RFID conveyor system

ABSTRACT

A conveyor system processes items on which radio frequency identification tags are disposed and includes a frame and a conveyor that is disposed moveably on the frame and that conveys items through a path of travel, each item having at least one respective radio frequency identification tag disposed thereon. An antenna is disposed on the frame with respect to the path of travel so that the antenna radiates radio frequency signals into a first area through which the items pass. The antenna comprises a substrate and a plurality of patch elements having respective generally planar surfaces and that are disposed on the substrate in respecting positions that are sequential with respect to a direction transverse to the path of travel. The generally planar surfaces of the patch elements are generally coplanar. The antenna includes a feed network that applies respective signals to each patch element that drive electric current at the patch elements to radiate the radio frequency signals. The respective signals applied by the feed network to at least two patch elements define a predetermined phase shift with respect to each other. A radio frequency transmitter drives the antenna to emit the radio frequency signals.

The present application claims priority to U.S. Provisional ApplicationNo. 60/666,938, entitled RFID Tracking System and filed Mar. 29, 2005,and U.S. Provisional Application No. 60/773,634, entitled RFID TrackingSystem and filed Feb. 15, 2006, the entire disclosure of each of whichis incorporated by reference herein.

BACKGROUND OF THE INVENTION

Most item tracking systems in commercial settings can be classified aseither a supply chain system or a manufacturing system. Manufacturingsystems generally track items that remain within a facility, whereassupply chain systems generally track items that move into, through andout of a facility. One common supply change system is a distributioncenter conveyor system on which boxes or other packages from a receivingarea are placed and transported to one or more stations at which thepackages are identified, sorted and distributed to appropriate locationsdepending on the sortation. It is well known in such systems to employbar code labels to the packages to thereby affix relevant information,for example manufacturer and package contents, that can be read bybarcode scanners disposed along the conveyor. The scanner outputsinformation to a controller that associates the information with thepackage and outputs the associated information to a host system that canthen manage the package's progression to its ultimate destination.

Barcode systems, although having developed to a high degree ofreliability, suffer from certain inherent limitations in that theyrequire a line of sight between the barcode scanner and the label, andthe barcode label must be applied on the package so as to be opticallyreadable. In contrast, radio frequency identification (RFID) tags do notrequire an optical line of sight for reading by an RFID reader.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one of ordinary skill in the art, is set forth moreparticularly in the remainder of the specification, which makesreference to the accompanying figures, in which:

FIG. 1 is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 2 is a functional block diagram of a tracking system for use withthe conveyor system as in FIG. 1;

FIG. 3 is a schematic illustration of an RF engine for use in a systemas in FIGS. 1 and 2;

FIG. 4 is a front schematic view of the conveyor system shown in FIG. 1;

FIG. 5 is a top perspective view of an RF antenna for use with theconveyor system as in FIG. 1;

FIG. 6 is a top perspective view of an RF antenna in accordance with anembodiment of the present invention;

FIG. 7 is an exploded view of an RF antenna in accordance with anembodiment of the present invention;

FIG. 8 is a schematic illustration of the RF antenna as in FIG. 7;

FIG. 9, which is presented as FIGS. 9A, 9B and 9C, is a schematic viewof a radiation pattern of the RF antenna as in FIG. 8;

FIG. 10 is a schematic illustration of the RF antenna as in FIG. 5;

FIG. 11, which is presented as FIGS. 11A and 11B, is a schematic view ofa radiation pattern of the RF antenna as in FIG. 10;

FIGS. 11C and 11D are graphical representations of components of theradiation pattern illustrated in FIGS. 11A and 11B;

FIG. 12, which is presented as FIGS. 12A and 12B, is a schematic view ofa radiation pattern from an RF antenna in accordance with an embodimentof the present invention;

FIG. 13 is a schematic view of a radiation pattern from an RF antenna inaccordance with an embodiment of the present invention;

FIG. 14, which is presented as FIGS. 14A and 14B, is a schematic view ofa radiation pattern from an RF antenna in accordance with an embodimentof the present invention;

FIG. 15 is a schematic view of a radiation pattern from an RF antenna inaccordance with an embodiment of the present invention;

FIG. 16, which is presented as FIGS. 16A and 16B, is a schematic view ofa radiation pattern from an RF antenna in accordance with an embodimentof the present invention;

FIG. 17 is an exploded view of an RF antenna in accordance with anembodiment of the present invention;

FIG. 18 is an exploded view of an RF antenna in accordance with anembodiment of the present invention;

FIG. 19 is a schematic illustration of an RF antenna for use with topand side antennas in a conveyor system as in FIG. 1;

FIG. 20A is a schematic illustration of a side antenna in a conveyorsystem as in FIG. 1;

FIG. 20B is a schematic illustration of a bottom antenna in a conveyorsystem as in FIG. 1;

FIGS. 20C and 20D are schematic illustrations of a top antenna in aconveyor system as in FIG. 1;

FIG. 21 is a perspective view of an antenna tunnel for use in a conveyorsystem as in FIG. 1;

FIG. 22 is a schematic illustration of a patch array for an RF antennain accordance with an embodiment of the present invention;

FIG. 23 is a schematic illustration of a patch array for an RF antennain accordance with an embodiment of the present invention;

FIG. 24 is a schematic illustration of an RF antenna in accordance withan embodiment of the present invention;

FIG. 25, which is presented as FIGS. 25A, 25B and 25C, is a schematicview of a radiation pattern from an RF antenna as in FIG. 24;

FIG. 26, which is presented as FIGS. 26A, 26B, 26C and 26D, is aschematic view of a radiation pattern of the RF antenna as in FIG. 24;

FIG. 27A is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27B is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27C is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27D is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27E is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27F is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 27G is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 28 is a functional block diagram of a tracking system for use withthe conveyor system as in FIGS. 27A-27G;

FIG. 29 is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 30, which is presented as FIGS. 30A, 30B, 30C and 30D, is aschematic view of the conveyor system as shown in FIG. 29;

FIG. 31, which is presented as FIGS. 31A, 31B, 31C and 31D, is aschematic view of antenna patch elements of the antennas in theembodiment as shown in FIG. 29;

FIG. 32 is a schematic view of a conveyor system in accordance with anembodiment on the present invention;

FIG. 33 is a functional block diagram of a tracking system for use withthe conveyor system as in FIG. 32;

FIG. 34 is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 35 is a functional block diagram of a tracking system for use withthe conveyor system as in FIG. 34;

FIG. 36 is a schematic view of a conveyor system in accordance with anembodiment of the present invention;

FIG. 37 is a functional block diagram of a tracking system for use withthe conveyor system as in FIG. 1;

FIG. 38 is a functional block diagram of a tracking system for use witha conveyor system in accordance with an embodiment of the presentinvention; and

FIG. 39 is a schematic view of the tracking system as in FIG. 38.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION

Reference will now be made in detail to presently preferred embodimentsof the invention, one or more example of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe invention, not limitation of the invention. In fact, it will beapparent to those skilled in the art that modifications and variationscan be made in the present invention without departing from the scope orspirit thereof. For instance, features illustrated or described as partof one embodiment may be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of thepresent disclosure.

Referring to FIG. 1, a conveyor system 10 includes a belt 14 and a frame12 that defines a generally planar horizontal top surface 13 extendingthe length of the conveyor and over which belt 14 carries a plurality ofsuccessive packages, e.g. boxes 22 and 24, in the belt's runningdirection (indicated at 28) from an upstream entry point indicatedschematically at 26. Top surface 13 may be comprised of a series ofrollers extending transverse to running direction 28, a planar metalsheet, or a combination of both. The specific construction of theconveyor frame and belt is not, in and of itself, a part of the presentinvention, nor is the present invention limited to any particular suchconstruction.

As described in more detail below, an RFID high speed controller (“HSC”)30 interfaces multiple RFID antenna engines and other devices intracking packages 22 and 24 as they progress down the conveyor. In thepresently described embodiment, HSC 30 is a PENTIUM equivalentmicroprocessor that is part of an industrial computer. HSC 30 runs theWINDOWS XP EMBEDDED operating system and is programmed with severalthreads developed under the WINDOWS.NET platform and written in theVB.NET language. It should be understood, however, that the organizationand operation of these threads as described herein is provided forpurposes of illustration only and that a control program may be writtenin any suitable language for operation on any suitable operating system.

A tachometer wheel (“TAC”) 32 is attached to frame 12 in contact withthe underside of conveyor belt 14 so that TAC 32 rotates with movementof belt 14 and outputs (to HSC 30 over a connection line 34) pulses thatcorrespond to the belt's linear movement over top surface 13. Becausethe pulse signals correlate in a consistent manner to movement ofconveyor belt 14 in direction 28, they are used by HSC 30 to selectivelytrack the locations of packages 22 and 24 along conveyor system 10.

A photodetector 36 is disposed on the frame next to the belt so that aline of sight is defined across and just above the belt betweenphotodetector 36 and a retroreflective sensor 37 disposed on the frameopposite the photodetector. Photodetectors and retroreflective sensorsshould be understood in this art and are available, for example, fromRockwell Automation/Allen-Bradley of Chelmsford, Mass. Because thephotodetector line of sight is perpendicular to the path of conveyorbelt 14, packages 22 and 24 traveling down conveyor system 10 interruptthe line of sight as they move in direction 28, thereby causing detector36 to output a first signal to HSC 30 via a connection line 38 at thepoint the package's leading edge enters the line of sight. Photodetector36 outputs a second signal to HSC 30 when the package's trailing edgepasses out of the line of sight. HSC 30 monitors the number of TACpulses between reception of the first and second signals and therebydetermines the length of the package. Although a photo-eye is describedherein with respect to the presently illustrated embodiments, it shouldbe understood that any other suitable presence detector, such as a lightcurtain or transfer plate system, for detecting an object may be used.

A second photodetector 40 is disposed on conveyor frame 12 at a distanceupstream from first photodetector 36 less than the length (in direction28) of the smallest-length package the conveyor is expected to carry.Second photodetector 40 and an opposing reflector 41 are attached toconveyor frame 12 so that a resulting line of sight is perpendicular tothe conveyor belt's path at a predetermined height above conveyor belt14 so that the photodetector detects the presence or absence of packagesof at least the predetermined height. When the leading edge of such apackage crosses the line of sight, second photodetector 40 outputs acorresponding signal to HSC 30 via a connection line 42. As describedbelow, the height of the second photodetector 40 line of sightcorresponds to the minimum height of packages it is desired to monitorby an overhead antenna. In presently preferred embodiments, the secondline of sight is 24 inches and 28 inches, respectively, above thesurface of conveyor belt 14, but this variable is configurable by theuser, and it should be understood that the user can set the heightvariable as desired for varying conditions. Furthermore, it should alsobe understood that second photodetector may be comprised of a singlephoto-eye disposed at the predetermined height, a light curtain having avariable output, or any other suitable presence detector capable ofoutputting a signal corresponding to package height. Where a singlephoto-eye is employed, the detector's binary signal indicates thepresence or absence of a package of the predetermined height. When alight curtain is used, however, the output varies with actual packageheight, and HSC 30 therefore determines whether a package of thepredetermined height has occurred through analysis of the light curtainoutput. Thus, the “line of sight” for a light curtain arrangement isdefined by the sensor output in combination with logic at the HSC.

An antenna frame 44 is disposed on conveyor frame 12 at a predetermineddistance downstream from first photodetector 36. As described in moredetail below, frame 44 defines an RFID antenna tunnel through whichpackages 22 and 24 travel for detection of RFID tags disposed on thepackages. Very generally, the tunnel is defined by a top antenna 46, apair of side antennas 56 and 66 and a bottom antenna 76, each of whichdefines a radiation pattern that extends from the respective antennatoward an area above belt 14 through which the packages travel.

A top antenna 46 is attached horizontally to the top of antenna frame 44so that the antenna is disposed directly above frame 12, spanstransversely across the path of belt 14 on the side of the belt on whichpackages are conveyed and is angled so that antenna 46, and thereforethe center of its radiation pattern, faces belt 14 at a 45 degree angle(with respect to a horizontal plane parallel to the conveyor belt) inthe downstream direction. In the illustrated embodiment, it is desiredto read tags on the packages only when the packages enter apredetermined detection zone, in this instance beginning at a line 49defined by the downstream edge of a series of radiation absorbent padsassociated with the top and side antennas. With respect to top antenna46, a stack of absorber pads 48 are attached to the top of antenna frame44 at the upstream edge of top antenna 46 so that absorber pads 48 aredisposed just below and close to the antenna. Absorber pads 48, whichare generally planar in shape and extend parallel to conveyor belt 14across the width of antenna 46, significantly shield the pattern andthereby inhibit antenna 46 from detecting upstream RFID tags.

A feed line 50 connects top antenna 46 to an antenna engine 52 thatdrives transmission signals to the antenna and that receives andprocesses signals received by the antenna. An output line 54 connectsantenna engine 52 to HSC 30.

Side antenna 56 is attached to the left side of frame 44 and laterallyoffset from the belt so that the antenna is oriented in a vertical plane(perpendicular to the horizontal plane in which belt 14 is disposed) ata 45 degree angle with respect to a vertical plane including theconveyor belt's centerline and so that antenna 56, and therefore thecenter of its radiation pattern, faces the conveyor belt at a 45 degreeangle (with respect to a vertical plane parallel to the belt's centerline) in the downstream direction. Antenna 56 is disposed sufficientlyhigh on the left side of antenna frame 44 so that the bottom of antenna56 is above the plane of conveyor belt 14. Absorber pads 58 are attachedto the left side of antenna frame 44 between antenna 56 and belt 14 sothat the pads extend from the antenna's upstream edge 45 downstream toline 49. Pads 58 extend parallel to a vertical plane including thebelt's centerline to a height at least equal the height of antenna 56.Similarly to pads 48, absorber pads 58 absorb radiation from antenna 56and thereby block at least a part of the antenna's upstream radiationpattern to thereby inhibit detection of RFID tags upstream of a lineextending transversely across the belt at the upstream edges of sideantennas 56 and 66.

A feed line 60 connects left side antenna 56 to an antenna engine 62that drives transmission signals to the antenna and that receives andprocesses signals received by the antenna. An output line 64 connectsantenna engine 62 to HSC 30.

Side antenna 66 is attached to the right side of antenna frame 44 sothat the antenna is oriented in a vertical plane (perpendicular to thehorizontal plane in which belt 14 is disposed) at a 45 degree angle withrespect to a vertical plane including the conveyor belt's centerline andso that antenna 66, and therefore the center of its radiation pattern,faces the conveyor belt at a 45 degree angle (with respect to a verticalplane parallel to the belt's center line) in the downstream direction.Antenna 66 is disposed sufficiently high on the right side of antennaframe 44 so that the bottom of antenna 66 is above the plane of belt 14.Absorber pads 68 are attached to the right side of antenna frame 44between antenna 66 and belt 14 so that the pads extend from theantenna's upstream edge 47 downstream to line 49. Pads 68 are ofidentical construction and orientation to pads 58. Similarly to pads 58,pads 68 absorb radiation from antenna 66 and thereby block at least apart of the antenna's upstream radiation pattern to thereby inhibitdetection of RFID tags upstream of the transverse line at the sideantenna's upstream edges.

A feed line 70 connects right side antenna 66 to an antenna engine 72that drives transmission signals to the antenna and that receives andprocesses signals received by the antenna. An output line 74 connectsantenna engine 72 to HSC 30.

A fourth antenna 76 is disposed in a horizontal plane below belt 14 sothat the antenna's radiation pattern extends upward and above the beltsurface. Antenna 76 is located directly between antennas 56 and 66 anddirectly below antenna 46. A feed line 78 connects bottom antenna 76 toan antenna engine 80 that drives transmission signals to the antenna andthat receives and processes signals received by the antenna. An outputline 82 connects antenna engine 80 to HSC 30.

Antennas 46, 56, and 66 are patch antennas that transmit and receive inthe range of 902 MHz to 928 MHz. Such patch antennas should beunderstood in this art, and it should be understood that various typesof antenna arrangements could be employed in the present system.Referring to FIG. 19, for example, each patch antenna 46, 56 and 66 iscomprised of three patch elements 410 disposed on a dielectric substrate412 on the opposite side of the substrate from a ground plane. A drivesignal to each patch is provided by a feed network 414 comprised ofrespective feed lines 416 that provide the drive signal to respectivesquare connectors 418. The side of each square patch is approximately5.15 inches, which is approximately one-half the wavelength of the drivesignal. The feed lines to each patch attach to adjacent sides of thepatch mid-way along the sides' lengths. The construction of the patches,and the materials comprising antennas 46, 56 and 66, are similar to thatof the construction of bottom antenna 76, which is discussed in detailbelow.

FIGS. 20A-20C schematically illustrate the disposition of the top, sideand bottom antennas with respect to the absorber pads and the conveyorbelt. FIG. 21 is a perspective view of an exemplary antenna tunnel.

Referring to FIG. 20A, side antenna 66 is disposed beside conveyor belt14 approximately 2 inches above the conveyor frame. A metal plate 67extends vertically upward from the conveyor and has an L-shapedcross-section in the X-Y plane. The length of each leg of metal plate 67is approximately 7.5 inches. The length (in the Y direction) of ferriteabsorber pads 68 is approximately 7.75 inches, such that the absorberpads extend slightly downstream of the downstream edge of metal plate67.

Side antenna 56 (not shown in FIG. 20A) is the mirror image of antennaconstruction 66, as shown in FIG. 21. A reference line 71 can beconsidered to extend transversely across conveyor belt 14 at the backedges of antennas 66 and 56. Upstream from this line (with respect tothe belt's running direction 28), there should be no detection of RFIDtags by the antenna tunnel. Moreover, absorber pads 58 and 68significantly reduce the antenna radiation pattern between referencelines 71 and line 49. Thus, while it is possible that an RFID tag may beread between lines 71 and 49, the absorber pads reduce this likelihood.

Referring to FIG. 20B, bottom patch antenna is disposed at a distance 73(approximately 8 inches) downstream from reference line 71. Thetransverse center line of patch elements 332 is a distance 75(approximately 7 inches) downstream of the leading edge of antenna 76,resulting in an approximately 15 inch downstream distance betweenreference line 71 and the line of patch elements 332.

Referring to FIG. 20C top antenna 46 is disposed so that its upstreamedge is aligned with reference line 71. Absorber pads 48 extend upstreamand downstream from antenna 46, the downstream edge of absorber pads 48being at a distance 77 (approximately 13.5 inches) from reference line71. Thus, the downstream edge of absorber pads 48 extend downstream fromline 49 (FIG. 20A). Referring also to FIG. 20D, absorber pads 48 aredisposed approximately 32 inches above the surface of conveyor belt 14.

As should be understood in the art, each of antennas 46, 56 and 66exhibit circular polarization in a plane parallel to the front surfaceof the antenna. That is, the antenna tends to strongly detect a dipoleantenna disposed in such a parallel plane but tends not to detect adipole antenna to the extent it is oriented perpendicular to such aplane. Because most packages expected to pass through the presentlydescribed RFID antenna tunnel embodiment are six-sided boxes withrelatively planar sides, the RFID tag antenna disposed on those sidesare likely to be disposed in vertical planes parallel or perpendicularto the longitudinal center line of conveyer belt 14 or parallel to thebelt's horizontal plane. Accordingly, while alignment of the side andtop antennas parallel and perpendicular to such planes would increasethe likelihood the antennas would detect some RFID tags, such anarrangement could eliminate one or more of the antennas from readingtags with perpendicular antennas, thereby possibly increasing thelikelihood the tag will not be read. To increase the possibility thatany one of the three antennas can detect a tag passing through thetunnel, the antennas are disposed at the 45° angles discussed above,although it should be understood that this angle may be varied.

While the present embodiment employs four radio frequency antennas, itshould be understood that other configurations and constructions arepossible. For instance, various types of RF antennas, in variousconfigurations, could be employed depending on conveyor system size andconfiguration, antenna cost, and RFID tag antenna variations.

Referring to FIGS. 1 and 4, a start window 84 (FIG. 4) is defined by aplane intersecting conveyor belt 14 at transverse line 49 (FIG. 1) andmarks the beginning of the most efficient antenna reading zone. Atransmit point 86 is defined by a second vertical plane transverse toand intersecting conveyor belt 14. The transmit point marks the locationat which HSC 30 saves information associated with a given package andremoves the information from a package queue. In the presently describedembodiment, transmit point 86 is located downstream from start window 84at a distance of or greater than twice the maximum package length (inthe belt's longitudinal direction) of a package expected to be carriedby the conveyor, plus the maximum spacing between packages. Forinstance, if conveyor system 10 is to convey packages up to 36 inches inlength at a between-package spacing of up to 15 inches, the minimumdistance (along the path of travel of belt 14) between start window 84and transmit point 86 is 87 inches.

Antenna engines 52, 62, 72, and 80 are identical. Each is comprised ofthe schematically illustrated components of FIG. 3, including amicroprocessor 100, connector 102, transmitter 104, receiver 106, andFPGA 108. Connector 102 connects transmitter 104 and receiver 106 to theantenna's feed line. Transmitter 104 and receiver 106 connect in turn toFPGA 108. Microprocessor 100 controls connector 102, transmitter 104,receiver 106 and FPGA 108 and communicates with HSC 30 via a connectionline 103. HSC 30 initiates a transmission from the antenna by a commandto microprocessor 100. In response, microprocessor 100 sends a bitsequence to transmitter 104, which then transmits the signal at aspecified frequency and power level to the antenna via connector 102 andthe antenna's feed line. The antenna returns a detected signal from anRFID tag to receiver 106 via the antenna's feed line and connector 102.Receiver 106 removes the carrier signal and sends the resultinginformation signal to FPGA 108. FPGA 108 extracts digital data from thereceiver's signal and outputs a resulting digital signal tomicroprocessor 100, which then transmits the digital data to HSC 30.RFID engines suitable for use in the presently disclosed system areavailable from AWID Wireless Informations, Inc. of Monsey, N.Y.; SymbolTechnologies, Inc. of San Jose, Calif. (e.g. the Matrics AR400); andThingMagic of Cambridge, Mass. (e.g. the MERCURY 4).

Conveyor system 10 may also include a scout reader 88 disposed betweenphotodetector 36 and antenna frame 44. In one preferred embodiment,scout reader 88 is identical in construction and arrangement to theantenna tunnel defined by antennas 46, 56, 66 and 76 and includes anantenna frame, top antenna, opposing side antennas, bottom antenna andrespective RFID engines as discussed above. For ease of explanation, theseparate feed lines from the engines of scout reader 88 (correspondingto feed lines 50, 60, 70, and 78) are indicated as a single connectionline 90 in FIG. 1. Similarly, the antenna engines of scout reader 88(corresponding to antenna engines 52, 62, 72, and 80) are indicatedcollectively as antenna engine 92, and the output lines (correspondingto output lines 54, 64, 74, and 82) are indicated collectively at outputline 94. Output lines 94 connect the scout reader antenna engines 92with HSC 30.

A barcode scanner 96 is disposed above belt 14 at a point betweenphotodetector 36 and transmit point 86. Barcode scanner 96 reads barcodelabels on packages 22 and 24 and outputs corresponding information toHSC 30 via a connection line 98. Barcode scanner 96 can comprise anysuitable bar code scanning system capable of reading barcode labels onpackages traveling at a speed defined by conveyor system 10 andoutputting corresponding information to a computer such as HSC 30, forexample as available from Accu-Sort Systems, Inc. of Telford, Pa.

As noted above, HSC 30 comprises a PENTIUM equivalent microprocessorrunning the WINDOWS XP EMBEDDED operating system and is programmed withseveral threads developed under the WINDOWS.NET platform and written inthe VB.NET language. Referring to FIG. 2, the software supported by HSC30 for operating the RFID tracking system described herein includes atracking thread 200, engine read threads 206, 208, 210 and 212, anantenna sequence generator 202, an antenna sequence thread 204, a hostthread 214, a socket thread 216, a user interface thread 218 and abarcode thread 220.

Tracking thread 200 is set to the highest priority defined by theoperating system and runs continuously until encountering a sleepcommand and thereafter entering a 1 millisecond (“ms”) sleep period.Antenna sequence thread 204 is also set to the operating system'shighest priority and has a dynamically-set sleep time that defaults to 2ms. Read engine threads 206, 208, 210, and 212 are set to above-normalpriority and sleep for 3 ms.

Tracking thread 200 handles communications with tachometer 32. As longas conveyor belt 14 moves in direction 28, TAC 32 constantly sendscorresponding pulse signals to HSC 30. Tracking thread 200 receivesthese signals and increments a global TAC variable 222 with the TACpulse id. That is, global TAC variable 222 is a running count of TACpulses from a defined start point. Referring also to FIG. 1, when belt14 moves a first package 22 so that it crosses into the photodetector 36line of sight, photodetector 36 immediately transmits a signal totracking thread 200 via connection line 38. At this point, package 22has entered the tracking system. Accordingly, upon receipt of thesignal, tracking thread 200 creates a package structure 228 with an idunique to the new package (unique, at least, with respect to otherpackages otherwise presently in the tracking system). The trackingthread stores in the package structure both the value of the global TACvariable at the point photodetector 36 detected the package and aninitialized (i.e., zero) local TAC value. That is, tracking thread 200identifies the package and identifies its starting position in thetracking system by a zero local TAC value, associating those two valuesin package structure 228. As the conveyor belt moves the package throughthe conveyor system, tracking thread 200 increments the package's localTAC value in its package structure 228 at each subsequent TAC pulse.Thus, the local TAC value in structure 228 represents the distance thepackage associated with the package structure has moved from the pointat which it first reached photodetector 36. Additionally, the sum of thepackage structure's local TAC variable and its initially stored globalTAC value (referred to herein as the “running TAC value”) can be used tocompare the associated package's position with respect to other eventsthat may occur downstream from photodetector 36.

Package structure 228 also includes a start-of-read window distance anda stop-read window distance. The start-of-read window represents the TACpulse distance between photodetector 36 and start window 84. At eachincoming TAC pulse, tracking thread 200 decrements the start-of-readwindow distance so that, at any given time between the point at whichthe leading edge of package 22 enters the photodetector 26 line of sightand the point at which the package's leading edge reaches start window84, the start-of-read window distance represents the number oftachometer pulses (the “TAC distance”) remaining between the package'sleading edge and start window 84. That is, the start-of-read windowdistance represents the distance package 22 needs to travel beforeentering the package's RFID detection zone.

Referring also to FIGS. 1 and 4, package structure 228 also includes astop-read window variable that defines the length of the detection zone,more specifically corresponding to a distance (again, preferably definedin TAC pulses) a package will travel beyond start window 84 during whichtracking thread 200 will correlate RFID information received from theantennas with the package and store such information in the associatedpackage structure 228. In other words, the stop-read window is thedistance downstream from start window 84 through which the package willpass while the tracking system attempts to read RFID tags that may beattached to the package. When the package first enters the trackingsystem at photodetector 36, tracking thread 200 initializes thestop-read window to a predefined default distance defined in TAC pulses.In the presently described embodiment, the default number of TAC pulsescorresponds to a distance of approximately 36 inches, although this is aconfigurable variable. Thus, unless the stop-read window changes asdiscussed below, the detection zone for the package will extendapproximately 36 inches downstream from start window 84.

Tracking thread 200 adds package structure 228 to a package queue 226,which maintains a running list of all packages presently in the trackingsystem between photodetector 36 and transmit point 86. In the presentembodiment, package queue 226 can store up to 500 package structures,although this limit is a configurable variable. Tracking thread 200 alsoupdates a queue flag 224—a global flag that indicates whether packagequeue 226 contains package information. If queue flag 224 is zero, nopackages are in the tracking system.

As package 22 continues to pass by photodetector 36, the photodetector'sline of sight remains blocked. When the following edge of package 22finally passes out of the line of sight, tracking thread 200 reads theTAC variable in the package structure at that moment. This valuerepresents the length of the package (in the direction of the conveyorbelt's movement), and tracking thread 200 stores the package length(plus a user-defined addition) in package structure 228.

If package 22 is taller than 24 inches (assuming the user has set theheight variable to 24 inches), it will have crossed the secondphotodetector 40 line of sight by the time it crosses the photodetector36 line of sight. If so, second photodetector 40 transmits a signal toantenna sequence generator 202 via connection line 42. The spacingbetween photodetectors 36 and 40 should be sufficiently short that theshortest package carried by the belt will be simultaneously detected bythe two sensors.

When a subsequent package (24) enters the photodetector 36 line ofsight, tracking thread 200 creates new package structure 230 in the samemanner as the creation of package structure 228 and adds the newstructure to package queue 226. As there is now a package following aprevious package (i.e., package 22) in the system, tracking thread 200checks the stop-read window variable for previous package 22 to confirmthat the distance between packages 22 and 24 does not require thevariable's reduction. As described above, the stop-read window variabledefines a package's detection zone from start window 84. If a subsequentpackage is sufficiently close to the previous package that it and theprevious package would be in the previous package's detection zone atthe same time, the possibility that the system may undesirably read RFIDtags from both packages and incorrectly associate one of the tags withthe wrong package is increased. Accordingly, if the subsequent packageis sufficiently close to the previous package, tracking thread 200reduces the previous package's stop-read window to exclude thesubsequent package from the previous package's detection zone.

More specifically, upon detecting the presence of subsequent package 24,tracking thread 200 adds the length of previous package 22 to thedistance between previous package 22 and subsequent package 24. Thedistance between the two packages is equal to the difference between theglobal TAC values stored in the respective package structures for thetrailing edge of package 22 and the leading edge of package 24. If thisvalue is less than the default stop-read window value, the packages aretoo close to maintain the existing stop-read window, and tracking thread200 resets the stop-read window value for previous package 22 to a newvalue equal to the length of previous package 22 plus one-half thedistance between packages 22 and 24.

In another preferred embodiment, upon detecting the presence ofsubsequent package 24, tracking thread 200 adds the length of previouspackage 22 to the distance between previous package 22 and subsequentpackage 24. If this value is less than the default stop-read windowvalue (e.g. 36 inches), and if the gap between packages 22 and 24 istwenty inches or more, tracking thread 200 resets the stop-read windowvalue for previous package 22 to a new value equal to the length ofprevious package 22 plus one-half the distance between packages 22 and24. If the value is less than the default stop-read window value, and ifthe gap between packages 22 and 24 is less than twenty inches butgreater than or equal to ten inches, tracking thread 200 resets thestop-read window value for previous package 22 to a new value equal tothe length of previous package 22 plus the length of the gap betweenpackage 22 and package 24, less ten inches. If the value is less thanthe default stop-read window value, and if the gap between packages 22and 24 is less than ten inches, tracking thread 200 resets the stop-readwindow value for previous package 22 to a new value equal to the lengthof previous package 22.

If the system includes scout reader 88, scout thread 219 constantlychecks queue flag 224 to determine when a package has been added topackage queue 226. If queue flag 224 indicates a package (assume package22) is present in the system, scout thread 219 activates scout reader 88via connection lines 94 to scout reader antenna engines 92. Scout thread219 activates scout reader 88 a predetermined number of TAC pulses afterthe package's detection to allow the package to reach the scout reader.This variable is user-configurable and can be set as low as zero,effectively activating the scout reader as soon as the package entersthe tracking system. The scout reader's engines do not read theinformation on RFID tags disposed on the packages, but instead onlydetermine the class of the RFID tags, which can be more rapidlyaccomplished. As should be understood in this art, determining the classof the RFID tags does not require the scout reader's engines to fullydecode the tags, but rather to simply analyze the received signal forRFID class. Antenna engines 92 transmit this information to HSC 30,where scout thread 219 receives and parses the signal and transfers theresulting information to antenna sequence generator 202, which is partof antenna sequence thread 204.

As should be understood in this art, there are presently two generaltypes of RFID tags—“class 0” tags produced by Matrics, Inc. and “class1” tags produced by Alien Technology. A “gen 2” tag is a modificationfrom class 0 not yet in wide use. Approximately 80% of present tags areclass 1. As should also be well understood, different protocols arerequired to read the different tags. Thus, in a default mode ofoperation in which the class of tags on packages passing through thedetection zone is unknown, engines 52, 62, 72 and 80 alternately queryfor class 0 and class 1 protocols to thereby assure either tag type isread. In the presently described embodiment, scout reader 88 does notassociate class type with particular packages, but because it outputs toHSC 30 signals describing the number of tags of each respective classpassing through the scout reader, it provides HSC 30 with sufficientinformation to determine the percentage of tags of each class passingthrough the tracking system over any given window of time. If tags inthe tracking zone are of only one class, HSC 30 can exclude the othertag's protocol from the engine queries, thereby increasing the system'sread speed and efficiency. In an alternate embodiment, if one tag typepredominates the other, HSC 30 can set the engines to issue acorrespondingly greater number of queries for that type, therebyincreasing the chance more tags will be read.

Antenna sequence thread 204 controls the RFID engines in transmittingRFID queries according to a sequence defined by antenna sequencegenerator 202. The antenna sequence generator defines the order in whichthe antenna sequence thread is to activate the antennas. In the presentembodiment, the sequence is defined by a list of id's associated withthe respective antennas. For example, if id's 1, 2, 3 and 4 respectivelyrefer antenna 56, 66, 46 and 76, a sequence of 1-2-3-4 causes antennasequence thread 204 to activate the antennas in that order. In thepresently described embodiment, antenna sequence generator 202 defaultsto such a round-robin sequence, except that if a package has a heightless than 24 inches, such that it is not detected by secondphotodetector 40, top antenna 46 (id 3) is eliminated from the sequence.It should be understood, however, that the present invention encompassesother sequences as desired.

Antenna sequence thread 204 constantly checks queue flag 224. If queueflag 224 indicates that a package structure is in package queue 226,antenna sequence thread 204 initializes antenna engines 52, 62, 72 and80, including setting the engines to operate at 57,600 baud. The antennasequence thread then requests an antenna sequence from antenna sequencegenerator 202 and instructs the RFID engines to drive the antennasaccording to the sequence. Since queue flag 224 is a binary value, thismeans that if there is at least one package anywhere in the trackingsystem, the antenna tunnel actively queries for RFID tags. The tunneldeactivates only when there are no packages in the queue, i.e., whenthere are no packages traveling along the conveyor system between firstphotodetector 36 and transmit point 86.

For each antenna present in the sequence, sequence generator 202 definesthe power level at which to drive the antenna during its activation, theclass of RFID tags to attempt to read during the antenna activation(i.e. read class 0, read class 1 or alternately read class 0 and class1), and the length of time the antenna is activated during its turn. Ina preferred embodiment, these parameters are defined as part of theround-robin sequence described above. In another embodiment, in whichantenna sequence generator 202 has received information from scoutthread 219 indicating that the packages in package queue 226 over apredefined look back period approximately equal to the time necessary tomove from photodetector 36 to start window 84 do not include one classof RFID tags or the other, antenna sequence generator 202 excludes thatclass from the antenna sequence instructions. The ability to exclude anentire RFID class improves read time and, therefore, the read rate andefficiency of the antennas' read capability. Otherwise, antenna sequencegenerator 202 includes an instruction in the antenna sequence requiringthat each antenna alternate between attempts to read class 0 and class 1tags. In one preferred embodiment, the sequence generator defines anantenna sequence requiring that each antenna sequence through attemptsto read class 0 tags, class 1 tags, and GEN2 tags. The particular tagprotocol is not, in and of itself, part of the present invention, andthose skilled in the art should understand that the system can beconfigured to operate with respect to different protocols. Accordingly,while the present discussion primarily provides examples of systems thatread class 0 tags and class 1 tags, it should be understood that this isfor example only and that the systems can query for and read GEN2 (orother protocol) tags in addition to or instead of class 0 tags and/orclass 1 tags.

The antenna sequence defines the power level at which each antenna isdriven based on tag class. As should be understood in this art, class 0tags have a stronger response than class 1 tags. Thus, it is desired todrive the antennas at a lower power level when reading class 0 tags toavoid over-driving the receiver, and the antenna sequence thereforeincludes an instruction requiring the antenna engines to reduce powerwhen querying for class 0 tags, although it should be understood thatthe antennas may be driven at the same power level for class 1 tags andclass 0 tags. Furthermore, because the position of the top antennafarther above the conveyor belt potentially enables the top antenna toread tags farther from the top antenna than the side antennas, the powerlevel at which the top antenna is driven is preferably less then theside antenna power levels.

Once determining (by checking the value of queue flag 224) that it isnecessary to activate the antennas, antenna sequence thread 204 obtainsa sequence from antenna sequence generator 202 and begins to execute thedefined sequence. Starting with the first antenna in the sequence,sequence thread 204 instructs the antenna's engine to set power to oneof the two levels and to read the corresponding tag class, as defined bythe sequence. Thread 204 then sleeps for 1 ms. Upon waking, antennasequence thread 204 issues a start reading command to the same antennaengine and then sleeps for the time required for the engine to read areturn signal received by the antenna from a tag of the predefined classtype. The sleep/read time in the presently described embodiment is 10 msfor a class 0 tag and 10 ms for a class 1 tag. If the sequence requiresthat the antenna tunnel query for both tag types, antenna sequencethread 204, upon waking from the sleep/read period, issues aninstruction to the same engine setting the power level for the other tagclass and then sleeps for 1 ms. Upon waking, sequence thread 204 issuesa start read command to the same engine and then sleeps for the timerequired for the engine to read a response from the antenna from a tagof the predefined class type. Having completed queries for both tagtypes at this antenna, or upon completion of the first query if only onequery is required, antenna sequence thread 204 issues a power offinstruction to the antenna's engine and then sleeps 3 ms to allow anyresponding RFID tag to fully discharge before being subsequentlyqueried.

Antenna sequence thread 204 then queries antenna sequence generator 202for a new sequence and repeats the process for the next antenna asdefined by that sequence. Antenna sequence generator 202 always ensuresthat antenna sequence thread 204 has the correct sequence of antennasand the correct tag class queued. Because sequence thread 204 gets a newsequence at each antenna read, the system can respond to sequencechanges from sequence generator 202 as they occur. For example, if atsome point there are no packages in the tracking system having a heightof 28 inches or more, when there had previously been such packages,sequence generator 202 eliminates the top antenna from the sequence. Inone preferred embodiment, antenna sequence generator 202 updates theantenna sequence upon detection of a package leaving the tracking system(i.e., passing transmit point 86).

In the presently described embodiment, the elimination or addition oftall packages to or from the tracking system is the only event thatcauses a change in the round robin antenna sequence. In anotherpreferred embodiment, however, antenna sequence generator 202 updatesthe sequence based on information stored by the system regarding thelevel at which each antenna performs valid reads and the prevalence ofone RFID tag class or the other. As described below, antenna readthreads 206, 208, 210 and 212 receive tag response information fromtheir respective RFID engines/antennas. As valid reads (as determinedand reported by the antenna engines) are received, the antenna threadsreport the information to tracking thread 200, which then storescorresponding information at 240 identifying the antenna id, the tagclass for each successful read, and the TAC value from TAC variable 222at which the read occurred. If the data at 240 indicates that a givenantenna has received a predefined greater number or percentage ofsuccessful reads than the others over a selectable predefined look backperiod, antenna sequence generator 202 weights that antenna's occurrencein the sequence correspondingly to its number of successful reads in thelook back period. Preferably, the look back period is defined as a rangeof global TAC values extending back from the present value of TACvariable 222 by a predetermined number of TAC pulses. Thus, antennasequence generator 202 can review stored tag read information at 240 forthose tag reads having associated global TAC values within the look backrange.

Additionally, if one class of RFID tag was read predominantly more thanthe other during the look back period, antenna sequence generator 202can set the instructions for successive reads for each antenna toreflect the weightings of the two classes. For example, if the system'sstatistical information at 204 shows that 85% of the successful readsduring the look back period were of class 0 tags and that the remaining15% were of class 1 tags, the antenna sequence generator can set theantenna sequence out for a predetermined period in the future (e.g.equal to the look back period) so that class 0 is queried at eachantenna activation (or, alternatively, at 85% of the activations overthat time period) but that both classes are queried at only 15% of thoseactivations.

Where a barcode scanner 96 is included in the system between firstphotodetector 36 and window start 84 (as discussed above, start window84 is defined by a plane intersecting the belt at transverse line 49shown in FIG. 1), barcode information received from barcode scanner 96may also be used in defining the antenna sequence instead of, or inconjunction with, scout reader information. As should be understood inthis art, barcode information typically identifies the product in thepackage and the product's manufacturer. If it is known that a givenmanufacturer applies tags of one class or the other to its packages,that class can be associated with the manufacturer in a predefined tableat 240, along with the TAC value from variable 222 at which the barcodeinformation was received. As barcode information is received by barcodethread 220 (which handles communications between HSC 30 and the barcodescanner) and passed therefrom to tracking thread 200, the trackingthread compares each manufacturer detected in the barcode informationwith the associated class for that manufacturer defined in the table.Thus, to the extent that manufacturers are associated with specificclasses of RFID tags, the detection of those manufacturers in thebarcode information is a detection of the associated RFID tag class.Accordingly, tracking thread 200 increments a count of tag classoccurrences based on detection of manufacturers from the barcodeinformation. Antenna sequencer 202 can therefore weight the antennaqueries between the two types of tags based on this information in thesame manner as described above with respect to the scout reader.

Generally, antennas 56 and 66 can read RFID information located on thetop of package 22 unless the height of package 22 is greater than theheight of the side antennas, in this example 24 inches. Because antenna46 is disposed at a relatively high position on frame 44, its radiationpattern extends farther downstream on the conveyor than the otherantenna patterns and may therefore be more likely to undesirably read atag on a downstream package that is otherwise beyond the package'sdetection zone. Thus, as noted above, antenna 46 is included in theantenna sequence in the present embodiment only where secondphotodetector 40 indicates a package is 24 inches high or taller. Morespecifically, sequence generator 202 adds the top antenna to thesequence when the start-of-read window variable for the identified tallpackage decrements to zero (i.e. when the tall package reaches startwindow 84) and removes the top antenna from the sequence when thepackage's stop-read window then decrements to zero (i.e. when thepackage reaches the end of its detection zone), or makes such changes atthe nearest respective points at which packages cross the transmit pointto thereby trigger a sequence update.

Accordingly, antenna sequence generator 202 possesses the ability tocreate independent power levels, frequency, and read times for any givenantenna, thereby allowing customization of the antenna sequence.

In a still further embodiment, it may be desired to always include allfour antennas in the antenna sequence. For example, where the conveyorsystem carries packages containing liquids or other materials that blockRF signals, RFID tags are sometimes readable only by the top antenna.Thus, even where package height is below the level that would otherwisetrigger activation of the top antenna in the sequence described above,in this embodiment the top antenna is always included in the round-robinsequence. In one preferred embodiment, the sequence generator emphasizesthe top antenna, defining a repeating five-step sequence for the fourantennas of (a) bottom antenna, (b) left antenna, (c) top antenna, (d)right antenna, and (e) top antenna. In the embodiment described below inwhich the bottom antenna is offset from the other three antennas and istherefore excluded from the round-robin sequence for these antennas, thesequence generator defines a four-step sequence for the three remainingantennas of (a) left antenna, (b) top antenna, (c) right antenna, and(d) top antenna. The user may select among the round-robin sequences(i.e. with top antenna control based on package height, with top antennaalways included regardless of package height, or with top antennaemphasis) through selection of a variable via remote system 244 andsockets thread 216.

As each antenna receives a power setting and a read command, it attemptsto read RFID tags present within its radiation pattern. As noted above,antennas 46, 56, and 66 are adjacent to absorber pads 48, 58, and 68,respectively, which inhibit antennas 46, 56, and 66 from readingupstream RFID tags. The operation and construction of antenna 76 isdescribed in more detail below, but the operation of the RFID enginesassociated antennas 46, 56, 66, and 76 is the same for all antennas andis therefore described only with respect to antenna 56 by way ofexample. During a read sequence, antenna 56 may detect responses fromRFID tags in its radiation pattern. The antenna then passes acorresponding signal to antenna engine 62 via feed line 60. Antennaengine 62 parses the received information, as described above, andtransmits the resulting digital data, along with an interrupt, to HSC 30via output line 64. Engine read thread 206 sees the interrupt, retrievesthe data, and assigns to it the present TAC value from global TACvariable 222. The resulting RFID information includes the RFID datareturned from the RFID tag (this data will generally be a tag numberunique to the tag and is referred to hereinafter as the RFID tag's“value”), the RFID tag class, the receiving antenna id, and the globalTAC value. Engine read thread 206 forwards the RFID information totracking thread 200.

Tracking thread 200 receives the posted RFID information and checks theRFID value against RFID tag values stored in a locked RFID collection236 and an old RFID collection 232. As indicated in the discussionbelow, if the RFID value of the newly-posted RFID information matches anRFID value in either locked RFID collection 236 or old RFID collection232, the RFID response is from a tag that has been previously read, andthe newly-posted tag information is therefore discarded. If the new tagvalue does not match the values in collections 236 or 232, trackingthread 200 checks a new RFID collection 234 to determine whether this isthe first instance the RFID value has been seen. If the RFID value isnew, tracking thread 200 adds the value to new RFID collection 234.

Whether or not the RFID value is new with respect to the list incollection 234, tracking thread 200 then checks each package structurein queue 226 and determines if there are any package structures forwhich (a) the start-of-read window value has decremented to zero as ofthe global TAC value associated with the newly-posted information and(b) the stop-read window value has not decremented to zero as of the TACvalue associated with the newly-posted information. If no packagestructure meets this criteria, no package was within its detection zone(i.e. the area beginning, and extending downstream from, start window 84within which the antenna radiation patterns extend and within which itis desired to read tags on the package) when the tag that provided thenewly-posted information was read. There is therefore no package towhich to associate the tag information at that time, and tracking thread200 places the newly-posted RFID information in a deferred RFIDcollection, or table, 238. If there is a package structure meeting thecriteria, a package (assume package 22) has reached start window 84 andwas within its detection zone as defined by the stop-read windowvariable when the responding tag was read. Tracking thread 200 thenchecks package structure 228 to determine whether any RFID informationhas been previously associated with package 22. If not, tracking thread200 associates the newly-posted RFID information with package structure228, waits for the next incoming RFID information and, when the nextsignal is received, repeats the procedure.

If there is RFID information associated with package 22 (i.e., RFIDinformation stored in package structure 228), tracking thread 200 checkshow many such records have previously been associated with the packagestructure. That is, the tracking thread checks to see if there have beenany previous RFID tag responses associated with this package. If thereis only one occurrence of previously associated RFID information, and ifthe RFID value of the previous information matches the RFID value of thenewly-posted RFID information, the system has now received twoconsecutive identical RFID tag responses for the same package, where noother tag responses have been received for that package. Tracking thread200 then determines if the RFID value of the first-received RFIDinformation was a new RFID value. If so, no previous package has beenassociated with this tag. Under these conditions, there is a highlikelihood that the received RFID tag information corresponds to a tagon the package associated with package structure 228, and trackingthread 200 therefore stores the RFID value in a locked variable definedin package 228 and assigns the RFID value to locked RFID collection 236.Since the RFID value of any subsequently received RFID information ischecked against the locked collection, no subsequent package will beassociated with the locked RFID tag value, even if the tag value isreceived in response to a subsequent query.

If the RFID value of the first-received RFID information was not a newvalue, then the same tag was read before the present package reached itsdetection zone. There is therefore an increased possibility that theresponding tag is not associated with that package. The tracking threadtherefore associates the newly-posted RFID information with the packagestructure, but does not put the RFID value in the package structure'slock variable or add the RFID value to the lock collection, and awaitsthe next incoming RFID information.

If there is only one previous occurrence of RFID information associatedwith package structure 228, but the RFID value for the previousinformation is different than the RFID value of the newly-postedinformation, then either (a) there may be more than one RFID tag on thepackage, or (b) at least one of the tag reads is from another package.In either event, the system has now received information reducing theconfidence that the RFID values now associated with the packagecorrespond to a tag that should be permanently associated with thatpackage. Thus, the tracking thread associates the newly-posted RFIDinformation with the package structure, without putting the RFID valuein the package structure's lock variable or adding the RFID value to thelock collection, and awaits the next incoming RFID information.

If there are multiple previous occurrences of RFID informationassociated with package structure 228, then (a) the package structure isalready locked to an RFID value, or (b) there are multiple differentRFID values already associated with the package that preclude a lock, or(c) there are at least two of the same RFID values that were associatedwith the package under conditions not justifying a lock. In any suchevent, the newly-posted RFID value cannot result in a lock, and thetracking thread therefore associates the newly-posted RFID informationwith the package structure, without putting the RFID value in thepackage structure's lock variable or adding the RFID value to the lockcollection, and awaits the next incoming RFID information. Accordingly,a package structure can collect multiple associated RFID valuesregardless whether it has a locked value.

As noted above, the presently described embodiment requires twoexclusive matching tag reads associated with a package in order todetermine that an RFID tag is disposed on the package and to therebylock that tag's RFID value to the package, thereby precluding the tag'sassociation with any other package. The number of such successive,exclusive occurrences required to lock the RFID value to packagestructure 228 is user configurable and can therefore be varied asdesired. Moreover, the present algorithm assumes that there can be onlyone valid tag on any given package, but it will be understood that it ispossible to vary the algorithm to allow one or more RFID values to lockon a package under different desired criteria, for example where it isknown that a package may support more than one valid RFID tag.

In the event a package passes through its detection zone without beingassigned any RFID information, there is the possibility that a tag onthe package was read just before the package reached start window 84 butnot thereafter. In that event, the tracking algorithm checks deferredRFID collection 238 and determines whether RFID information was receivedwithin a predetermined period prior to the point the package entered itsdetection zone. If so, the tracking algorithm associates thisinformation with the package, although without locking any correspondingRFID value. More specifically, when both the start-of-read and stop-readwindow variables for any package structure reach zero (i.e. when thepackage has passed through its detection zone), tracking thread 200checks the RFID information associated with the package structure. Ifthere is any RFID information associated with the package structure,then valid tag reads have been received for this package. As there islikely less confidence for tag reads in the deferred collection than forthose tag reads occurring while the package was in the detection zone,it is less desirable to rely on the earlier tag reads, and the deferredtag reads are not associated with the package. Further, since thehighest likely association of any RFID information presently in thedeferred collection is to the package now leaving the detection zone,there is no need to further retain such information if it is not to beassociated with that package. Thus, under such circumstances, trackingthread 200 removes all RFID values stored in deferred RFID collection238.

If, however, there is no RFID information associated with the packagestructure as the associated package leaves its detection zone, trackingthread 200 determines whether any RFID information stored in deferredRFID collection 238 was posted within a set number of TAC pulses priorto the global TAC pulse at which the package reached start window 84. Ifso, tracking thread 200 associates this RFID information with packagestructure 228 but does not lock any RFID value. Tracking thread 200 thenremoves all RFID values stored in deferred RFID collection 238. In thepresently described embodiment, the deferred collection look back periodis five TAC pulses, but this variable is configurable and can thereforebe set as desired.

If the system includes a barcode scanner 96, and if barcode scanner 96reads a barcode located on package 22, the barcode scanner outputs asignal to HSC 30 that is received by barcode thread 220. Barcode thread220, which is set to an operating systems priority below the otherthreads discussed herein, checks for information received from barcodescanner 96 and associates with it the current TAC value from the globalTAC variable 222. Barcode thread 220 transmits this information totracking thread 200, which compares the barcode TAC value with therunning TAC value of each package. If there is a package structure(assume package structure 228) corresponding to a package with a localTAC value equal to the TAC pulse distance between barcode scanner 96 andphotodetector 36 when the package structure's running TAC value is equalto the global TAC value associated with the barcode information, plus orminus a predetermined tolerance (e.g. 1 TAC pulse), tracking thread 200stores the barcode information in package structure 228, therebyassociating the barcode information with package 22.

When tracking thread 200 determines that the local TAC variable of anygiven package structure (assume package structure 228) increments to avalue equal to a stored value corresponding to the distance betweenphotodetector 36 and transmit point 86 (i.e. package 22 reaches transmitpoint 86), tracking thread 200 once again checks the RFID informationassociated with package 22. Tracking thread 200 compares each unlockedRFID value stored in package structure 228 with the RFID informationassociated with the immediately subsequent (or, upstream) package 24(stored in package structure 230). If any RFID value stored in packagestructure 230 matches any RFID value stored in package structure 228,tracking thread 200 creates confidence ratings 229 and 231 for eachcommon RFID value. Confidence rating 229 is equal to the occurrences ofthe given RFID value associated with package 22 as a percentage of thetotal occurrences of the RFID value associated with packages 22 and 24.Similarly, confidence rating 231 is equal to the occurrences of thegiven RFID value associated with package 24 out of the total occurrenceof the RFID value associated with packages 22 and 24. Tracking thread200 retains the RFID information for the RFID value in association withthe package structure having the higher confidence rating and removesthe corresponding RFID information from the other package structure. Ifthe confidence ratings are equal, the RFID information is retained inthe latter package structure (230) and removed from the earlier packagestructure (228). Once every RFID information record in package structure228 has been compared to the RFID information records in packagestructure 230, tracking thread 200 adds each RFID value remaining inpackage structure 228 to old RFID collection 232. Package structure 228is then added to stored information 240 and removed from package queue226. Tracking thread 200 performs the same routine for each package thatreaches transmit point 86.

If package queue 226 thereafter empties of all package structures,tracking thread 200 instructs antenna sequence thread 204 and scoutthread 219 to forward an instruction signal to the RFID engines of themain antenna tunnel and the scout tunnel, respectively, to power downall antennas. After a set number of TAC pulses following the power downinstruction, tracking thread 200 removes all RFID values stored in newRFID collection 234, old RFID collection 232 and locked RFID collection236.

In other preferred embodiments, the tracking thread may apply anadditional confidence rating to the package structure when thecorresponding package reaches transmit point 86. In one embodiment, adefault confidence rating of 1,000 is applied to each package structureupon its creation. When a package reaches the transmit point, trackingthread 200 reduces the initial confidence rating depending on theoccurrence of any of five conditions. That is, for each condition thatis present, tracking thread 200 subtracts a number from the defaultvalue (i.e. 1,000) that is associated with the particular condition. Thevalues, or weightings, associated with each condition can be set by theuser via remote system 244 or GUI 246. By assigning a zero weighting toany given condition, that condition is ignored in defining theconfidence rating.

Condition 1. When the package reaches the transmit point, trackingthread 200 reviews the package structure stored at 240 for the previous(i.e. downstream) package. If that package had no RFID informationassociated with it when it passed the transmit point, there may bereduced confidence that RFID information associated with the presentpackage was transmitted by a tag on the present package. That is, theremay be a higher likelihood that the RFID information originated from atag on the previous package. Accordingly, a user-defined negativeweighting may be applied to the default confidence value. Additionally,the tracking thread may also check the package structure of the nextupstream package in package queue 226 and, if either the downstream orupstream package has no tag reads, reduce the confidence rating for thepackage at the transmit point.

Condition 2. Depending on system conditions such as belt speed, packagesize and package spacing, there may be an increased likelihood that RFIDresponses may be incorrectly associated not only with an adjacentpackage, but also with the next package downstream or upstream from theadjacent package. Accordingly, when a package reaches the transmitpoint, the tracking thread may also check the stored information at 240for the second-previous package passing the transmit point (and,optionally, the second-upstream package in package queue 226) and, ifthat package (or, either package) has no associated RFID informationassociated with it at the transmit point, apply a user-defined negativeweighting to the confidence level for the present package.

Condition 3. As described above, where two adjacent packages have beenassigned common RFID information, tracking thread 200 associates theinformation with the package having the greater number of reads.However, even though the system has determined to which package theinformation should be associated, the fact that the information wasassociated with both packages to begin with reflects a probability thatthe tag information is incorrectly assigned that is higher than if theinformation had been associated with only one package. Accordingly, whenthe package reaches the transmit point, the tracking thread reviews thepackage structures for the upstream packages in package queue 226. Ifany package structure has been assigned a tag value that is alsoassigned to the package at the transmit point, the tracking threadlowers the confidence rating for the package at the transmit point by auser-defined negative weighting. As noted above, if the next previouspackage structure includes a tag value in common with the package at thetransmit point, the tracking thread allocates the tag value to thepackage having the greater number of reads and removes the tag valuefrom the other package structures in the package queue. Thus, if anupstream package structure in package queue 226 has a tag in common witha package at the transmit point, there is a likelihood that the tagvalue will be removed from the upstream package structure as a result ofthe tracking algorithm comparison, thereby preventing application of thenegative weighting to the upstream package's confidence rating based onthe common tag when the package later reaches the transmit point.Accordingly, when the tracking thread reduces the confidence rating fora package at the transmit point on the grounds that there exists atleast one upstream package structure with the same tag value, thetracking thread also reduces the confidence rating for the upstreampackage structure. When the upstream package structure eventuallyreaches the transmit point, the tracking thread may reduce the package'sconfidence rating if it shares other common tag values with upstreampackage structures, or, alternatively, the algorithm may be programmed(e.g. by examination of a flag in each package structure set when theconfidence rating is reduced based on common tag values) to reduce theconfidence rating for any given package only once for this condition.

Condition 4. As described above, if RFID tag information is receivedwhen no package is within its detection zone defined by the start/stopread variables, the tag information is assigned to the deferred group.If the information is thereafter removed from the deferred collectionwithout application to a package, there may be diminished confidence ina later association of the same RFID information with a package.Accordingly, when RFID information is removed from the deferredcollection, tracking thread 200 stores the tag values at 240. If apackage reaching the transmit point includes RFID informationcorresponding to a tag that has been removed from the deferredcollection and not assigned to a package (i.e. those tags removed fromthe deferred collection and stored at 240), the tracking thread appliesa user-defined negative rating to the confidence level for the packageat the transmit point.

Condition 5. If a conveyor system carries packages on which it isexpected there to be only one RFID tag, the association of multiple RFIDtag information reduces the confidence that the correct tag has beenassociated with the package. Accordingly, if multiple RFID taginformation is associated with a package at the transmit point, trackingthread 200 applies a user-defined negative weighting to the confidencelevel for the package at the transmit point.

The confidence rating is stored with the package structure and may beutilized by downstream systems to make decisions about the package. Forexample, a sortation system may require at least a minimum confidencerating to process packages. Thus, upon receiving the package structurefrom HSC 30, the sortation system checks the confidence rating and, ifit is below a user-defined threshold, diverts the package for specialhandling.

In a still further embodiment, tracking thread 200 locks RFID values asdescribed above, but, after assigning an RFID value to the lockedcollection, nevertheless assigns the same (i.e. locked) RFID value tothe same or subsequent packages if RFID information with the now-lockedvalue is received when the same or subsequent packages are in thedetection zone. When the tracking thread determines that the local TACvariable of any given package structure (assume package structure 228for package 22) increments to a value equal to a stored valuecorresponding to the distance between photodetector 36 and transmitpoint 86 (i.e. package 22 has reached transmit point 86) and checks theRFID information associated with package 22, the tracking threadcompares each locked RFID value stored in package structure 228 with theRFID information associated with the immediately subsequent (i.e.upstream) package 24. If any locked RFID value stored in packagestructure 228 is also associated with package structure 230 for package24, tracking thread 200 creates respective confidence ratings for eachsuch common RFID value. The confidence rating for package structure 228is equal to the occurrences of the given locked RFID value associatedwith package 22 as a percentage of the total occurrences of that RFIDvalue associated with packages 22 and 24. Similarly, the confidencerating for package structure 230 is equal to the occurrences of thatRFID value associated with package 24 out of the total occurrencesassociated with packages 22 and 24. If the confidence rating for package24 (i.e. the subsequent package) is greater than the confidence ratingfor package 22 (i.e. the package at the transmit point) by a multiple of2.5 or more, tracking thread 200 assigns the RFID information to packagestructure 230 for package 24 and removes the information from packagestructure 228 for package 22 and all other package structures in queue226. The tag value is stored in old RFID collection 232, therebypreventing its association with subsequent packages. If the confidencerating for package 24 is less than 2.5 times the confidence rating forpackage 22, the RFID information remains with package 22. The tag valueis removed from package structures in package queue 226 and added to oldcollection 232.

Host thread 214 supports a list of host interface protocols and allowsHSC 30 to send tracking information to a host computer 242 via anEthernet or serial connection. Host thread 214 can transmit packagetracking information to host computer 242 from stored information 240.Host thread 214 is set to the lowest priority and will be interrupted byany of the other higher priority threads that require processor time.The threshold level (2.5, in this example) used to compare theconfidence ratings for locked tags can be configured by the user throughGUI 246 or remote system 244.

Socket thread 216 allows remote connection to the system for remotemonitoring and testing. User interface thread 218 provides a user withinformation from the system including, but not limited to, reports andstatistics of total packages, total packages with RFIDs, total packageswith barcodes, total packages with both, and total packages withoutRFIDs. Graphical user interface (“GUI”) 246 is a web based interface andis preferably written in JAVA or other platform-independent languagethat allows GUI 246 to run on any system. Socket thread 216 and userinterface thread 218 are set to the lowest priority and are interruptedby any of the other higher priority threads that require processor time.

A user or technician can connect to the system from GUI 246 or remotesystem 244 to make changes to several of the system parameters discussedabove. The user may set through GUI 246 the respective power levels forreading class 0 and class 1 RFID tags, the read time through which eachengine will transmit a query signal from each antenna in the sequence(and, therefore, the time antenna sequence thread 204 sleeps prior toreading the response), the time for which an engine will attempt to reada response (and, therefore, the corresponding time the sequence threadwill sleep), and whether the sequence generator thread can include inthe sequence only class 0 tags, only class 1 tags or both. GUI 246 sendsthese parameters to user interface thread 218, which then forwards themto antenna sequence generator 202 for consideration in generatingsubsequent antenna sequences.

A user may change the following variables via remote system 244 tosockets thread 216: (a) the number of consecutive, exclusive new RFIDvalues required to be received before an RFID value is locked to apackage; (b) the number of TAC pulses that must occur before trackingthread 200 removes all RFID information from old RFID collection 232,new RFID collection 234, and locked RFID collection 236 followingemptying of queue 226; (c) the number of inches (in TAC pulses) to addto the length of each package from the length determined byphotodetector 36; (d) the start-of-read window value; (e) the defaultstop-read window value; (f) the TAC pulse distance between barcodescanner 96 and photodetector 36; (g) the number of TAC pulses to be usedas the tolerance level in associating barcode information to an RFIDpackage structure; (h) the number of TAC pulses between photodetector 36and transmit point 86; (i) the number of pulses the tachometer willoutput per inch; (j) sequence option; (k) confidence rating weightings;and (l) the threshold level for comparison of confidence ratings forlocked tags. Remote system 242 transmits these parameters to socketsthread 216, which forwards the information to tracking thread 200.

Host thread 214, sockets thread 216, and user interface thread 218 canretrieve stored information 240. This information can be used to displaystatistics on RFID class, percentages of each antenna used, barcodeassociations, and success rate of the RFID system. Display statisticscan apply to specific packages (e.g. associated RFID tag values,barcodes, tag class, antenna reads, and confidence levels) as well as torunning count statistics for the tracking system (e.g. number of RFIDs,number of barcodes, the number of combined RFID tags and barcodes, andthe number of no reads). Host thread 214, sockets thread 216, and userinterface thread 218 can display these statistics via host computer 242,remote system 244, and GUI 246 respectively.

As noted above, with reference to FIG. 1, conveyor system 10 includes abottom antenna 76 that defines a radiation pattern extending aboveconveyor belt 14. Referring also to FIG. 5, bottom antenna 76 iscomprised of a patch array antenna disposed in a frame 302 havingforward and rear ramps 306 and 304. With reference also to FIGS. 6 and18, which illustrate patch array antennas in similar frames, frame 302also includes a bottom pan 308 made of a planar sheet of stainless steeland received between ramps 304 and 306 and side rails 310.

Frame 302 is secured to the conveyor system frame immediately under theconveyor belt by screws (not shown) received through holes 312 in frontand rear ramps 306 and 304 and that screw into the conveyor frame'splanar top surface 13 (FIG. 4). The antenna frame therefore creates aslight rise as the belt moves over forward ramp 306, across patch arrayantenna 76 and down rear ramp 304. Due to the antenna frame's lowprofile, however, the rise is slight, and in a preferred embodiment thevertical height of the frame and antenna combination, measured from theconnection between ramps 304/306 and the top surface of the antenna, isabout 0.8 inches. It should be understood, however, that the antennaframe may be dimensioned as desired, provided the frame/antennaassembly's height does not impair progress of the packages on theconveyor system.

Side rails 310 are received in grooves defined at the corner of frontand rear ramps 304 and 306. A 0.5 inch ultra-high molecular weight(UHMW) polyethylene cover 309 secures the patch antenna array within theramps and the rails and is secured to the ramps and the rails by aplurality of screws.

The bottom antenna may be disposed on the conveyor in a frame withoutramps. Referring to the embodiment shown in FIG. 7, for example, bottompan 308 is bounded by a rim 314 secured by screws or rivets to siderails 310. Rim 314 is raised from the surface of pan 308 so that the rimand pan define a depression that receives patch array antenna 76. Aplurality of flanges 318 are stamped and bent upwardly from the body ofpan 308, and pins 320 are disposed on the planar surface of pan 308between the flanges. Flanges 318 and pins 320 are received incorresponding slots and holes 322 and 324 in patch array antenna 76 tosecurely orient antenna 76 in the pan's depression. Cover 309 isdisposed over the top of antenna 76 and secured to rails 310 by flanges326 received in respective longitudinal grooves 328 in rails 310.

In each of the above-mentioned embodiments, the antenna framedepression's depth, and the thickness of patch array antenna 76, isapproximately 0.75 inches, so that the top surface of the antenna isapproximately flush with the top of rim 314 (FIG. 7) or rails 310 (FIGS.5, 6 and 18).

Furthermore, the forward and rear ramps may include or be replaced byrollers or other suitable structure. Referring to FIG. 17, for example,front and back ramps 304 and 306 define longitudinal grooves 380 and382, respectively, that receive axles 384 and 386 on which are disposedaxially aligned rollers 388 received in respective notches 390 in theramps. Antenna 76 is received in respective slots 392 (only one of whichis shown in FIG. 17) in frames 304 and 306 to secure the antenna inposition with respect to the ramps. Slats 394 extend over antenna 76 andare secured at slots 396 in ramps 304 and 306 by screws 398. Theconveyor belt passes over the assembled frame such that the belt'smovement is facilitated by the rollers.

While the above-described embodiments of antenna frame 302 may bepresented with an antenna having a given patch element arrangement, itshould be understood that this is for purposes of example only and thatthe antenna arrangements disclosed herein can be housed in any of thevarious frames.

Patch array antenna 76 is comprised of a low permittivity polymer foamsubstrate 330 and a copper ground plane (not shown) bonded to thesubstrate's underside. An exemplary substrate/ground plane material isFOAMCLAD 100, available from Arlon Microwave Materials Division ofArlon, Inc., of Bear, Del.

In the embodiments illustrated in FIGS. 5, 7, 8, 10 and 18, patch arrayantenna 76 includes a single row of three patch elements 332 on the sideof substrate 330 (see, e.g. FIG. 7) opposite the ground plane. Eachpatch is stamped from approximately 0.0014 inch thick copper or otherhigh-conductivity metal to form a 5.15 inch sided square and is disposedin the substrate so that the top of the patch is flush with the topsurface of the substrate.

As noted in more detail below, however, the antenna may have more orfewer than three patch elements, for example depending on the frequencyband at which the antenna will operate and the physical space of theconveyor belt. Systems in accordance with the present invention mayemploy antennas having two or four, or other suitable number of, patchelements.

Referring to the embodiments illustrated in FIGS. 5 and 18, a coaxialconnector 334/336 connects the antenna PCB board 76 to an RF cable fromthe respective RF engine. In the embodiment shown in FIG. 7, coaxialconnector 334/336 is connected to the antenna assembly by pins 340.

In all the illustrated embodiments, feed network 338 is a corporatenetwork that combines the power received from each patch element anddelivers the combined signal to center conductor 342 along separatepaths from respective patch elements 332. Each path includes two feedlines 346 and 348 attached mid-way along adjacent sides of the patchelement. Feed lines 346 and 348 are attached at their opposite ends toadjacent top corners of a square connector 350 comprised of sides of alength approximately one-quarter the wavelength of the signal carried bythe feed network. A trace 352 extends from a first bottom corner ofsquare connector 350 and is connected to ground through a resistor. Theopposite bottom corner of the square connector is connected to the feedtrace.

The feed line extending from square connector 350 has an impedance of130 ohms, whereas the initial feed line extending from an entry point344 has an impedance of 150 ohms. Accordingly, a one-quarter waveelement may be disposed within the feed trace to match the impedances.The patch elements' impedance varies with frequency, and the elementsdefine an impedance giving an acceptable impedance match only over arelatively small percentage of the radiation bandwidth. Of course, therange of what is considered an acceptable impedance match may depend onthe performance required of an antenna in a given system. As should bewell understood in this art, several things affect a patch array'sachievable bandwidth. Chief among these are dielectric thickness anddielectric losses between the patch elements and ground. Accordingly,these characteristics may be varied as desired to achieve a desiredimpedance match and operative frequency range. In the presentlydescribed embodiments, patch array antenna 76 operates within afrequency range of 902 to 928 MHz, as dictated by the FederalCommunications Commission. The feed network and patch elements areconstructed and arranged so that there is les than −15 dB return loss.

It should also be understood that the antenna construction andarrangement may otherwise vary. For example, the patch elements maydefine shapes other than squares.

Assuming the center of the 902 MHz to 928 MHz operative bandwidth, or915 MHz, the antenna's center wavelength (in air) is approximately 13inches. As should be understood in this art, however, the permittivityof the substrate and cover material reduces the wavelength of the drivesignal in the antenna from the in-air wavelength, the two wavelengthsbeing related by a factor of the in-air wavelength divided by the squareroot of 2.3, and in the illustrated embodiment, the antenna wavelengthis approximately 10.3 inches. As noted above, the length of each side ofeach square patch element 332 is one-half the antenna wavelength, andthe length of each side of each square connector 350 is one-quarter theantenna wavelength: Accordingly, the side of each patch element 332 isapproximately 5.15 inches, and the length of each side of squareconnector 350 is approximately 2.58 inches.

The patch elements are aligned in a row extending transverse (the Xdirection) to the path of the conveyor belt so that the center patch 332is disposed in the center of the belt's path. The side patches arealigned with the center patch in the transverse direction, and thedistance from patch corner 366 to patch corner 368 is approximately 26inches, or approximately the width of the conveyor belt. Thecenter-to-center spacing between adjacent patches is approximately nineinches.

In operation, and referring initially to the embodiments shown in FIGS.6, 7 and 8, center conductor 342 applies the drive signal to entry point344 so that the drive signal is then applied by feed traces 333 to thebottom left corners of the respective square connectors 350. Because thelength of each leg of the square connector is one-quarter of the drivesignal wavelength, each leg introduces a one-quarter phase shift in thesignal. That is, for example, the drive signal measured at squareconnector corner 360 lags the signal measured at corner 362 by ninetydegrees, and the signal measured at corner 364 lags the signal measuredat 360 by ninety degrees. Thus, and because the lengths of feed lines346 and 348 are equal, the drive signal applied to patch element 332 byfeed line 346 leads the signal at the connection between the patchelement and feed line 348 by 90°.

As should be understood by those skilled in this art, each patch elementresonates primarily at its edges, and the 90° phase shift in the signalsat feed lines 346 and 348 causes the electric current pattern in thepatch elements to rotate in the counterclockwise direction.

Because the corresponding feed lines 333, square connectors 350 and feedtraces 346/348 for each patch element 332 of the array embodiment shownin FIGS. 6, 7 and 8 are of identical lengths and arrangement, theelectric current patterns in the patch elements rotate in the samedirection, and the patches' radiation patterns are in phase. Referringalso to FIGS. 9A, 9B, and 9C, the antenna arrangement shown in FIGS. 6,7 and 8 produces a radiation pattern 370, in which the X directionextends transverse to the path of the conveyor belt, the Y directionextends longitudinally to the belt's path (i.e. the X-Y plane is theconveyor belt plane), and the Z direction extends vertically above thebelt.

Radiation pattern 370 is the radiation pattern's far field electriccomponent. As should be understood in this art, the far field can beconsidered as the area outside a sphere of a radius equal to twice thesquare of the antenna array's longest dimension (in this instance, 26inches), divided by the in-air wavelength, where the patch array isconsidered to be a point. While there is a transition area between thenear and far fields, the radiation pattern in the near field area isdominated by the electric field component. Particularly when above orotherwise very close to a patch in the near field, an RFID tag isdetected by the patch's near field component without interference fromthe other patches. Generally, it is desired that the RFID tags respondto the near field pattern, but not the far field pattern, to therebyreduce the likelihood of an undesired response from an RFID tag on apackage other than the desired package.

Again referring to FIGS. 9A-9C, radiation pattern 370 has a main lobehaving a relatively wide dimension in the Y axis along the conveyorbelt's center line. The size of the two side lobes is generally afunction of the spacing between the patches and is minimized when thecenter-to-center spacing between the patches is equal to one-half thedrive signal's in-air wavelength. Given the preferred dimensions ofpatch elements 332 in the presently described embodiments, the nine inchcenter-to-center spacing was chosen to extend across the approximately26 conveyor belt span, thereby resulting in the side lobes illustratedin FIGS. 9A-9C.

As shown in the figures, the radiation pattern extends from the patcharray upstream and downstream in the Y direction with respect to theconveyor belt's path of travel and above the belt in the Z direction sothat the front and back edges of the radiation pattern extend at anangle in the Y-Z plane. Accordingly, RFID tags disposed on relativelylow packages carried by the conveyor belt may not impact the far fieldradiation pattern until relatively close to bottom antenna 76. Tags ontaller packages, however, may impact the main lobe farther before andfarther after the antenna. Thus, depending on the spacing between thepackages, there may be an increased probability that the antenna willsimultaneously receive responses from different RFID tags disposed ondifferent packages that are simultaneously within the radiation pattern.

To reduce the far field pattern, and thereby reduce the probability thatresponses from tags on different packages will be simultaneouslyreceived by the antenna array, the phase of the drive signal applied tothe individual patches in the antenna array is shifted at the respectivepatches to a predetermined relationship with respect to the drive signalphase at the other patches. Referring to the embodiment shown in FIGS. 5and 10, for example, patch array antenna 76 is comprised of threepatches 332 a, 332 b, and 332 c constructed identically to the patchesdescribed in the patch array with respect to FIGS. 6, 7 and 8. Squareconnectors 350 a, 350 b, and 350 c and feed traces 346 a/348 a, 346b/348 b and 346 c/348 c are also of the same construction as thecorresponding components in the embodiment of FIGS. 6, 7 and 8. The feednetwork between entry point 344 and the square connectors, however, ischanged both to shift the drive signal phase to the respective patchelements and to alternate the electric current pattern rotations. Morespecifically, feed line 333 b has the same length as in the arrangementof FIGS. 6, 7 and 8 and attaches to the same corner 362 b of its squareconnector. Feed line 333 a however, is shortened by one-quarterwavelength and now attaches to a lower right hand corner 372 a of squareconnector 350 a. Corner 362 a is now grounded. Similarly, corner 362 cof square connector 350 c is grounded, whereas feed line 333 c nowconnects to corner 372 c.

Because feed line 333 a is shortened by one-quarter wavelength withrespect to feed line 333 b, the phase of the drive signal presented topatch element 332 a at its square connecter is shifted by −90° withrespect to the drive signal presented to patch element 332 b at itssquare connector. On the other hand, feed line 333 c is increased by aquarter wavelength with respect to feed line 333 b, and the phase of thedrive signal presented to patch element 332 c at its square connector352 c is shifted by a +90° with respect to the drive signal presented to332 b.

Because the length of the feed network between entry point 344 and patchelement 332 a differs from the length of the feed network between entrypoint 344 and patch element 332 c by a half wavelength, and because feedlines 333 a and 333 c are connected to the bottom right hand corners oftheir respective square connectors, the electric current patterns inpatch elements 332 a and 332 c rotate in the same direction, and thesetwo patch elements are always 180° out of phase. Because feed line 333 bremains connected to the lower left hand corner of its square connector,however, the current pattern on patch element 332 b (which is 90° witheach of the other two patches) rotates in the opposite direction to thatof the two adjacent patches. That is, the current pattern rotationaldirection alternates among adjacent patch elements.

FIGS. 11A and 11B illustrate the radiation pattern resulting from theantenna configuration shown in FIGS. 5 and 10 in the X-Y plane and X-Zplane, respectively. As illustrated particularly in FIG. 11A, theradiation pattern has two main lobes that diverge to either side of theY axis. Since the Y axis lies longitudinally along the center of theconveyor belt, this means that the main lobes extend away from thecenter of the belt, that the far field radiation pattern along thebelt's center is smaller as compared to the center of the far fieldpattern shown in FIGS. 9A-9C, and that the gain of the far field patternalong the belt center is less than the far field pattern gain at thesame position in FIGS. 9A-9C (approximately 2 dB in FIG. 11A, as opposedto approximately 13 dB as shown in FIG. 9B). To the extent RFID tags arepositioned toward the sides of the belt, the gain of the two main lobesas shown in FIG. 11 is reduced about 3 dB from the main lobe of thepattern shown in FIG. 9. Thus, both in terms of gain and spatialorientation, the far field pattern of the antenna arrangement shown inFIG. 10 is less likely to detect an RFID tag than the arrangement shownin FIGS. 7 and 8.

FIGS. 11C and 11D illustrate another aspect of the antenna arrangementshown in FIG. 10 that reduces the probability the far field radiationpattern will detect an RFID tag from the probability applicable to theantenna arrangement shown in FIG. 8. As should be understood in thisart, the radiation pattern shown in FIG. 9 from the antenna arrangementof FIG. 8 is circularly polarized in the far field. That is, the farfield radiation pattern shown in FIG. 9 can detect an RFID tag at anyorientation in the horizontal (X-Y) plane. The radiation pattern doesnot detect vertically oriented tags. The alternating rotationaldirections of the current patterns on the patch elements shown in FIG.10, however, result in contributions to the overall radiation patternthat are linearly polarized.

Referring again to FIG. 10, for example, the radiation patterns of patchelements 332 a and 332 c tend to cancel each other in the far fieldbecause these two patch elements are 180° out of phase. Thus, the totalradiation pattern can be considered a combination of contributions fromelement 332 a and 332 b, on the one hand, and element 332 b and 332 c,on the other. Referring to FIGS. 11C and 11D, the total gain pattern 276(taken in the X-Z plane) is comprised of a linearly polarized firstpattern 278 and linearly polarized second pattern 280 that is offset 90°in polarization with respect to first pattern 278. More specifically,the difference in current pattern rotational direction between patchelements 332 a and 332 b results in a gain pattern exhibitingpolarization in the Y direction according to pattern 280, whereas thedifference in current pattern rotation between patches 332 b and 332 cresults in a gain pattern that is polarized in the X direction accordingto pattern 278. Thus, to the extent an RFID tag passes through the farfield radiation pattern of the antenna arrangement shown in FIG. 10 at apoint at which the tag is perpendicular to the field's polarization inthat area, the tag may not be detected. In other words, the radiationpattern shown in FIGS. 11A and 11B is not uniform in its polarization inthe horizontal plane, and it is therefore possible that an RFID tag maypass through the far field pattern undetected, depending upon itshorizontal orientation and the path taken through the field. Thus, as aresult of its different polarization characteristics, the far fieldpattern shown in FIG. 11 may be less likely to detect RFID tags than isthe pattern shown in FIG. 9.

Referring again to FIGS. 5, 6, 7 and 18, the placement of antenna 76 andframe 302 between the conveyor frame and belt generally providesinsufficient space for placement of radiation absorbing materials thatmight otherwise block the antenna's far field radiation pattern. Thus,the reduction of the far field pattern through phase shifting of thedrive signal applied to the respective patch elements reduces theeffectiveness of the far field pattern in generating RFID tag responsesand thereby reduces the likelihood of undesired tag reads.

In view of the present disclosure, it should be understood that variousmodifications can be made to the antenna arrangement shown in FIG. 10while still improving the antenna's far field performance over anarrangement such as shown in FIG. 8. Assume, for example, that thelengths of feed traces 333 a, 333 b and 333 c remain the same as in FIG.10 but that the feed lines are attached to corners 362 a, 362 b, and 362c of square connectors 350 a, 350 b and 350 c respectively. Theresulting radiation pattern is illustrated in FIGS. 12A (X-Y plane) and12B (X-Z plane). As with the pattern shown in FIG. 11, the radiationpattern illustrated in FIG. 12 is relatively narrow in the Y directionat the center of the conveyor belt. Although the gain of the main lobeis not significantly reduced from the gain of the main lobe shown inFIG. 9, the lobe on the right hand side of the belt is relatively small.

Still referring to FIG. 11, the width of the antenna radiation patternin the Y direction can be further reduced through the use of a doublerow of patch array elements. That is, two parallel rows, each havingthree patch elements in the same configuration as shown in FIG. 10, aredisposed adjacent to each other and extend transversely across the spanof the conveyor belt. Thus, two patch elements 332 a are aligned in theY direction, two patch elements 332 b are aligned in the Y direction,and two patch elements 332 c are aligned in the Y direction. Referringto a frame of the type shown in FIG. 7, for example, the dual antennamay be accommodated by extension of bottom pan 316 in the Y direction. Athird rail 310 is disposed between the two adjacent antennas 76 suchthat flanges 326 of the adjacent antennas are received in correspondinggrooves 328 on opposite sides of the third rail. Cover 309 is alsoextended in the Y direction to cover both antennas. Connector 336 isreplaced by a splitter that splits the drive signal provided by centerconductor 342 to respective feed lines that connect to the respectiveantenna entry points.

Although additional parallel rows may be added to the arrangement tofurther narrow the far field radiation pattern, the number of such rowsmay be limited by the resulting increase of the near field detectionarea. That is, because the near field detection zone is increased witheach antenna row, an increase in the number of antenna rows couldactually increase the likelihood of undesired simultaneous RFID tagreads from different packages, depending on the spacing between packageson the conveyor. Accordingly, the number of additional rows willgenerally depend upon the configuration of the particular system.

Similarly, the number of patch elements in a single row in the Xdirection may also depend upon the width of the conveyor belt. Anapproximately 26 inch wide conveyor belt assumed in the examplesdescribed above favors the three-patch configuration at the 902 MHz-928MHz operating range. Where patch arrays are used with belts of differentwidths or in different frequency ranges, however, and/or where patchesotherwise differing in their dimensions are used, more or fewer patchelements may be used within a transverse row. In such circumstances,phase shifted drive signals may be applied to the patches to improve farfield performance.

Referring to FIG. 22, for example, assume that a patch array used undera narrower belt (e.g., approximately 20 inches) has two 5.15 inch squarepatch elements 450 extending transversely across the width of the belt.Assume also that the feed line (similar to feed line 333 in FIG. 8) tothe square connector of one patch element is one-half wavelength longerthan the feed line to the other patch element and that the feed linesboth connect to the bottom left hand corner of the respective squareconnectors. If feed line 452 is considered at −180°, then feed line 454is at −90°, feed line 456 is at 0°, and feed line 458 is at 90°. Thatis, the patch elements are 180° out of phase, and the patches' electriccurrent patterns rotate in the same direction. As indicated by theresulting radiation pattern illustrated in FIG. 13, the width of the farfield radiation pattern is reduced at the belt center line, and the twomain lobes have reduced gain.

Referring to FIG. 23, assume now a wider conveyor belt (e.g.,approximately 40 inches) and an antenna array comprised of four 5.15inch square patch elements 460 arranged side-by-side with acenter-to-center spacing of nine inches. FIGS. 14A and 14B illustratethe resulting radiation pattern when feed lines 462 and 470 areconsidered at −180°, feed lines 464 and 472 are at −90°, feed lines 466and 474 are at 0°, and feed lines 468 and 476 are at 90°. That is, thedrive signals are applied to the patch elements so that the patchelements of each pair of adjacent elements are 180° out of phase and allpatch elements have the same current pattern rotation. FIG. 15 alsoillustrates a four-element radiation pattern. In this instance, feedlines 462 and 476 can be considered at −90°, feed lines 464 and 466 areat 0°, feed lines 468 and 470 are at 90°, and feed lines 472 and 474 areat 180°. Thus, the patch elements of each adjacent pair of elements are90° out of phase, and all patches have the same current patternrotation. FIGS. 16A and 16B illustrate a radiation pattern when feedlines 462 and 466 can be considered at 0°, feed lines 464 and 476 are at−90°, feed lines 468 and 472 are at 90°, and feed lines 470 and 474 areat 180°. Thus, the first and third patch elements, and the second andfourth patch elements, are 180° out of phase, and the current rotationof patch elements one and three is opposite to the current patternrotation of patch elements two and four.

As described above, the default sequence generated by antenna sequencegenerator 202 (FIG. 2) includes instructions to activate antennas 46,56, 66, and 76 (FIG. 1) one-by-one in a round-robin sequence. Inaccordance with this default sequence, antenna sequence thread 204activates each antenna via its corresponding antenna engine individuallyand sequentially, thereby avoiding interference that might occur ifantennas were activated simultaneously. At each antenna, the sequencethread first drives the engine to query for one of the two standard tagprotocols (i.e. class 0 or class 0.1) and then the other. That is, thereare two query/receive cycles for each antenna. Thus, if all fourantennas are activated for a given package, eight reads are required tofully query for a tag.

Depending on the speed at which the system moves packages, the distancebetween packages, and the rate at which the system reads tags, there maybe insufficient time to execute eight tag queries. For example, assume apackage moves at 600 feet/minute, that 0.104 seconds is needed toexecute all eight reads, that the minimum length of a package handled bythe system is six inches, and that the minimum distance between twopackages is fifteen inches. As noted above, the system defines thedetection zone as a distance from the start window equal to a package'slength plus one-half the distance between sequential packages. In oneembodiment, however, if the gap between packages is greater than orequal to ten inches, and less than twenty inches, the detection zonedistance equals the package's length plus the distance betweensequential packages, less ten inches. Thus, the minimum detection zone,and therefore the shortest distance over which a tag in the system underthis configuration might be available to be read, is 11 inches. At 600ft/min, the tag would be available for reading only for 0.0917seconds—less time than is needed to execute all eight reads. Further, ina preferred embodiment the system desirably makes two attempts to readand confirm a tag. With a tag available for only 0.0917 seconds, thesystem can make, at best, only one attempt to read a tag.

Moreover, because tags on package bottoms pass so closely to the bottomantenna, the bottom antenna's beam width may not extend the detectionzone's full width when reading such tags. In one preferred embodiment ofa bottom antenna as discussed herein, for example, the antenna beamwidth between the antenna patches immediately above the conveyor beltand in the direction of the conveyor's movement is approximately 4.25inches (beam width reaches approximately ten inches at the patches).Thus, and again assuming the package moves at 600 ft/min, a bottom tagwould be optimally readable by the bottom antenna only for 0.0354seconds. This is less than the time needed to cycle through the eightantenna queries and therefore increases the chance a bottom tag will notbe read.

Accordingly, in one preferred embodiment of the present invention, andreferring to FIG. 32, bottom antenna 76 is disposed upstream, withrespect to the belt's path of travel, from the other three tunnelantennas as shown in FIG. 1. Bottom antenna 76 is activatedcontinuously, i.e. without interruption to allow for activation of thedownstream antennas. Thus, the bottom antenna queries for tagssimultaneously with each of the downstream antennas as they areactivated in sequence. Accordingly, the bottom antenna should cyclethrough the two reads (where only class 1 and class 2 tags are read)during the time when a tag is optimally readable by the bottom antenna.

More specifically, the downstream edge of the printed circuit board onwhich the patch elements of antenna 76 are disposed is fourteen inchesupstream from the upstream edges of absorber pads 48, 58 and 68 as shownin FIGS. 20A and 20C. In the presently described embodiments, the bottomantenna printed circuit board is 18 or 20 inches in length, and thepatch elements in the bottom antenna are disposed approximately 43-45inches upstream from their non-staggered position with the other threeantennas as shown in FIGS. 1 and 20B. In one embodiment, the length ofthe PCB in the Y direction (FIGS. 20A-20C) is twenty inches where rampsare provided in the frame, and eighteen inches otherwise. It should beunderstood, however, that the length of the upstream shift of antenna 76may vary as desired, provided undesired RF interference betweensimultaneously activated antennas in the system is avoided. Morespecifically, as described below, responses are received from RFID tagsby the staggered antenna as the tag passes through an area proximate theantenna into which the antenna radiates its query signals. That is,there is an area proximate the antenna in the path of travel throughwhich the RFID tag travels as it moves on the belt and in which it isdesired that the tag receive and respond to RF signals from the antenna.This area may be considered a local detection zone for the staggeredantenna and preferably is at least as large as the detection zonedefined for the downstream antennas from which the staggered antenna isoffset. In one embodiment, the area is tied directly to the size of thedownstream detection zone by a predetermined offset applied to responsesreceived by the staggered antenna.

Considering the areas proximate the respective antennas in which it isdesired to identify RFID tags passing therethrough to be “detectionzones” for the respective antennas, the separation of simultaneouslyactivated antennas is preferably sufficient so that signals from onesimultaneously activated antenna do not undesirably inhibit reception ofand response to RF query signals of any other simultaneously activatedantenna by an RFID tag in the detection zone of the other simultaneouslyactivated antenna. Radiation from one simultaneously activated antennamay extend into the detection zone of another simultaneously activatedantenna, and it is therefore possible that some degree of interferencemay occur in the respective detection zones. The extent to which suchinterference is acceptable may depend, for example, on the performancelevel required by the user of the conveyor system, and the separationdistance may be chosen based on system testing. With the antennas spacedat various distances, the system may be tested until an acceptable readrate is achieved using simultaneously activated antennas and/or untilthe system receives an acceptably low number of simultaneous reads fromthe same tag by staggered, simultaneously activated antennas. Theseparation distance may further depend, for example, on the type andconfiguration of any RF shielding employed around the antennas tendingto reduce interference, the power at which the antennas are driven, theantennas' radiation patterns, and the environment around the antennas.If the system is operated in an area in which RF reflective surfacesexist relatively closely about the antennas, for instance, it may bedesirable to further separate the simultaneously activated antennas toreduce the likelihood that a first antenna will receive a signal from anRFID tag responding to a query from a simultaneously activated secondantenna. Accordingly, the spacing between staggered antennas in a givenconfiguration may depend on one or more of these several factors asapplicable.

As in the embodiment described above with respect to FIGS. 1 and 2, RFIDengine 80 is controlled by HSC 30 to drive bottom antenna 76. Whilerespective engines 62, 72 and 52 (FIG. 1) may again be employed to driveantennas 56, 66 and 46 in the tunnel downstream from staggered bottomantenna 76, in the presently described embodiment a single engine 700drives the three aligned tunnel antennas. In operation, both enginesdrive their antennas in the same manner as described above. HSC 30initiates a transmission from antenna 76 by a command to the engine'smicroprocessor, which in turn sends a bit sequence to the engine'stransmitter for driving the antenna at a specified frequency and powerlevel. If a response signal is detected and returned by the antenna, theengine's receiver removes the carrier signal and sends the resultinginformation to the FPGA for extraction of digital data to themicroprocessor and on to HSC 30.

Antenna engine 700 is similar in construction and operation to theengines described above with respect to FIGS. 1 and 2. The antennaengine has a single port from which an output feed line drives the threetunnel antennas through a radio frequency switch 702. To drive aparticular one of the three antennas, HSC 30 simultaneously outputs afirst signal to antenna engine 700 defining the power and frequency ofthe RF signal to be sent by the selected antenna and a second signal toantenna switch 702 to thereby select the particular antenna to which theswitch will direct the engine's driving signal.

In the alternative, antenna engine 700 may include multiple ports bywhich the engine respectively drives antennas 46, 56, and 66. While theengine preferably drives the ports and antennas with a singletransmitter/receiver pair, it should be understood that the engine mayinclude a transmitter/receiver pair for each antenna. In addition toantenna power and frequency, HSC 30 provides to the engine'smicroprocessor the identification of the antenna to be so activated, andthe microprocessor selects the appropriate port.

As described in the earlier embodiment with respect to FIGS. 1 and 2,depending on a package's height, antenna sequence generator 202 includesall four RF tunnel antennas in the antenna sequence. By default, onepreferred sequence requires activation of each antenna to query andreceive responses for one tag class and then the other in a round-robinsequence. Referring to FIGS. 1 and 2, for example, antenna sequencethread 204 in the default sequence instructs antenna engine 52 to powerantenna 46 on, set its power level and frequency to transmit to andreceive signals from class 0 RFID tags, transmit an RF query signal, andread any corresponding returned signals. Antenna sequence thread 204then instructs antenna engine 52 to change the power level and frequencyof antenna 46 to query and receive responses from class 1 tags. Asdetermined by the antenna sequence, antenna sequence thread 204 theninstructs the next antenna engine to attempt to read class 0 and class 1through its antenna in the same manner.

In the presently described embodiment, however, bottom antenna 76remains on continuously, whereas the antennas (46, 56, and 66) thatremain in their original positions in the RF antenna tunnel continue toalternate in the round-robin sequence described above. Referring also toFIG. 33, antenna sequence thread 204 instructs engine 700 to driveantennas 46, 56, and 66 and instructs antenna switch 702 to select eachof the three antennas in turn and in conjunction with the instructionsto engine 700. Because bottom antenna 76 is disposed upstream from theRF antenna tunnel a sufficient distance so that RF tags on conveyor belt14 should not receive and respond to transmissions from the downstreamantenna tunnel and bottom antenna 76 at the same time, the bottomantenna continuously transmits and receives RF signals without effectiveinterference with or by the other antennas.

A second sequence generator 704 defines a sequence that includes an idand corresponding instructions only for bottom antenna 76. A secondantenna sequence thread 706 constantly checks queue flag 224. If queueflag 224 indicates that a package structure is in package queue 226,second antenna sequence thread 706 initializes antenna engine 80 andrequests a sequence from second antenna sequence generator 704. Becausethe sequence from second sequence generator 704 includes only antenna76, second sequence thread 706 continuously activates the bottom antennaas long as a package structure is in the queue. Second sequencegenerator 704 defines the power level at which to drive the antennaduring its activation, the class of RFID tags to attempt to read duringthe antenna activation (for example, read class 0, read class 1 or readclass 0 and class 1), and the length of time the antenna is polled (i.e.the length of time for a query and response cycle). The poll time may beselected as part of the system configuration. In one embodiment, thedefault poll time is thirteen milliseconds.

Any RFID information received by antenna 76 during read periods istransmitted via antenna engine 80 to a read thread 212 at HSC 30. At HSC30, tracking thread 200 assigns a variable to the RFID information equalto the global TAC variable 222 plus the number of TAC pulses thatcorrespond to the distance by which antenna 76 is displaced upstreamfrom the position it would otherwise have occupied in the RF tunnel. Thetracking thread then stores the variable and the associated RFIDinformation in a FIFO structure 710. FIFO 710, which is a data structureknown as a first-in-first-out stack that should be well understood inthe art, is maintained by tracking thread 200 but is illustratedseparately in the figures for purposes of explanation. The trackingthread thereafter compares the value of global TAC variable 222 witheach variable stored in FIFO structure 710 at every incoming TAC pulse.The FIFO variable located at the top of the stack (i.e. the variablestored in the stack the longest amount of time) has the lowest value andwill be the first to equal global TAC variable 222. When a FIFO variableequals global TAC variable 222, tracking thread 200 removes the variableand the associated RFID information from FIFO structure 710 andprocesses the RFID information in the same manner as if it had receivedthe RFID information at that time from any of the downstream tunnelantennas via read thread 708. The FIFO variable associated with the RFIDinformation is treated as the TAC value for that RFID information whenthe algorithm considers RFID information read while a package is in thedetection zone. The tracking thread may consider each RFID informationas it is received, or the tracking thread may accumulate all RFIDinformation as it is received while a package is within the detectionzone (i.e. while the package's start-of-read window variable hasdecremented to zero but its stop-read window variable has not), orderthe RFID information according to its associated TAC values when thepackage's stop-read window decrements to zero, and then consider eachRFID information in turn, by associated TAC value, according to thealgorithm described above with respect to FIGS. 1 and 2. Because thevariable assigned to the RFID information stored in FIFO 710 correspondsto the value the global TAC variable would have if bottom antenna 76 hadreceived the RFID information when the package carrying the transmittingRFID tag were in the same position with respect to bottom antenna 76were bottom antenna 76 in the position it would have occupied if it werealigned with the downstream tunnel antennas, the RFID information fromFIFO 710 is considered in turn as if the tag from which the RFIDinformation was received had been read by an antenna aligned with thedownstream tunnel antennas.

The delay in providing the received tag information to tracking thread200 corresponds to the travel of the RFID tag (and, therefore, thepackage on which it is disposed) from (a) the position at which the tagactually transmitted the received signal to (b) the position from whichthe tag would have transmitted the signal were the bottom antennaaligned in the tunnel as described with respect to FIG. 1. Accordingly,tracking thread 200 processes RFID tag information from the bottomantenna relative to other RFID information transmitted by antenna readthread 708 as if bottom read antenna 76 was not offset from the RFantenna tunnel. Thus, upon receiving any RFID tag information, whetherfrom the top, side or bottom antennas, tracking thread 200 processes andstores the information according to the same algorithm as if the fourantennas were aligned.

Because the tag information received from the offset antenna isconsidered by the algorithm with respect to the same start/stop-readwindow differential, the TAC variable assigned to the RFID informationin FIFO 710, and the resulting delay, define the length of the localdetection zone for the staggered antenna to be the same as the length ofthe common detection zone for the downstream antenna(s). The delaytherefore causes the tag responses to be treated as if the detectionzones for all the tunnel antennas coincided.

It should be understood that other suitable methods of associating RFIDinformation received from the staggered bottom antenna with theappropriate package may be employed. For example, rather than assigningthe sum of the global TAC variable and a TAC value equal to the distancebetween the staggered bottom antenna and the position the bottom antennawould have occupied in alignment with the downstream antennas, trackingthread 200 may assign a variable to the RFID information equal to thenumber of TAC pulses that correspond to the distance by which antenna 76is displaced upstream from the other tunnel antennas. Tracking thread200 then stores the variable and the associated RFID information in aFIFO structure 710. The tracking thread decrements each variable storedin FIFO structure 710 by one TAC value at every incoming TAC pulse.Accordingly, the FIFO variable located at the top of the stack, whichwill also be the variable stored in the stack the longest amount oftime, has the lowest value. When the variable at the top of the stack isequal to zero, tracking thread 200 removes the variable and theassociated RFID information from FIFO structure 710 and processes theRFID information in the same manner as if it had received the RFIDinformation at that time from the tunnel antennas via read thread 708.

Still further, the system may be configured to define a local detectionzone, and associated start and stop read window variables, with thestaggered bottom antenna and to associate RFID information received fromthe bottom antenna with package structures for packages passing throughthe bottom antenna's local detection zone in the same manner asdiscussed above with respect to the common detection zone for theantenna tunnel as in FIG. 1. However, when RFID information is assignedto a package structure based on reception of the RFID information fromthe staggered bottom antenna, an offset variable may be assigned to thestored RFID information equal to the offset (in TAC pulses) between thestaggered bottom antenna and the position the bottom antenna would havemaintained in alignment with the downstream antennas so that theantennas maintain a common detection zone. The tracking threaddecrements this variable with each incoming TAC pulse and, when thevariable decrements to zero, re-processes the RFID information in thesame manner as if it had received the RFID information at that time fromthe tunnel antennas via read thread 708.

In the embodiment described with respect to FIGS. 1 and 2 above, antennaengine read threads 206, 208, and 210 receive and process RFID taginformation from antenna engines 62, 72, and 52 before sending theinformation to tracking thread 200. In the presently-describedembodiment, the functions of antenna engines 62, 72, and 52 areconsolidated into antenna engine 700, and the functions of antenna readthreads 206, 208, and 210 have, likewise, been consolidated to a singleantenna read thread 708. Any RFID tag information received by antennas46, 56, and 66 is transmitted to antenna read thread 708 by antennaengine 700. Antenna read thread 708 transmits the information totracking thread 200, which processes this information in a manneridentical to the algorithm described above with respect to FIG. 2.

In a still further preferred embodiment, and referring also to FIGS. 34and 35, two of the three antennas remaining in alignment with thedownstream RF antennas in the embodiment of FIG. 32 are offset upstreamfrom their tunnel positions in FIG. 32 and set apart from one another adistance sufficient so that RFID tags on conveyor belt 14 should notreceive and respond to transmissions from any two antennas at the sametime. As described above, the distance by which each antenna is movedupstream from the RFID antenna tunnel and separated from the otherantennas depends on several factors and may vary so long as the spacingacceptably avoids undesired RF interference between antennas. Theparticular spacing in a given configuration may be determined by spacingthe antennas at various separations, running packages through thestaggered antenna group, recording the results and determining whetherresponses from a single RFID tag are received by multiple antennas and,if so, whether the number and/or rate of such multiple reads areacceptable in the environment in which the system operates.

In the embodiment as shown in FIG. 34, the bottom antenna is againdisposed 43-45 inches upstream from its position shown in FIG. 1. Thedownstream edge of side antenna 56 is disposed fourteen inches (or othersuitable distance as determined by system testing) upstream from theupstream edge of bottom antenna 76, and the downstream edge of sideantenna 66 is disposed fourteen inches (or other suitable distance asdetermined by system testing) upstream from the upstream edge of sideantenna 56. As indicated in FIG. 34, side antennas 56 and 66 are notdisposed at a 45 degree angle with respect to the belt but are, instead,disposed facing the belt in a plane parallel to the belt's direction oftravel. As noted above, the 45° angled orientation of the antennas tothe belt centerline may improve the likelihood of tag reads,particularly where the belt carries packages generally defining sixplanar sides aligned parallel or perpendicular to the path of travel,but a flush alignment such as shown in FIG. 34 is also suitable, forexample including where the belt carries items of irregular shape and/ororientation on the belt, such as airline luggage.

RF absorber pads may be omitted. In one preferred embodiment, however,respective absorber pads are disposed adjacent to and immediatelyupstream and downstream from each side antenna. Top antenna 46 remainsdisposed at an angle with respect to the belt, as described above withrespect to FIG. 1. Where absorber pads are provided, L-shaped metalplates may be provided that extend vertically upward from the conveyor.Ferrite RF absorber pads are disposed on respective plates (and may bedisposed on the front and back of each plate) and extend slightlyupstream or downstream of the edge of metal plate, depending whether thepad is upstream or downstream, respectively, of the antenna.

The separation among the antennas allows the system to drive theantennas simultaneously and continuously, and, preferably, respective RFengines 72, 52, 62 and 80 drive antennas 66, 46, 56 and 66. Referringalso to FIG. 35, engines 52, 80, 62 and 72 are controlled by respectivesequence threads 204, 706, 712 and 714 and output received RFIDinformation to respective read threads 716, 212, 718 and 720.

Each of the sequence generators defines a sequence that includes an idand corresponding instructions only for its respective antenna. Thecorresponding sequence thread constantly checks queue flag 224. If queueflag 224 indicates that a package structure is in package queue 226 (seeFIG. 2), the antenna sequence thread initializes its correspondingantenna engine and requests a sequence from the antenna sequencegenerator. Because the sequence includes only the single correspondingantenna, the sequence thread continuously activates the antenna as longas a package structure is in the queue. The sequence generator definesthe power level at which to drive the antenna during its activation, theclass of RFID tags to attempt to read during the antenna activation (forexample, read class 0, read class 1 or read class 0 and class 1), andthe length of time the antenna is polled for each read.

Each of the three staggered antennas functions in the same manner asdescribed above regarding bottom antenna 76 in FIGS. 32 and 33. When HSC30 receives any RFID information from any of the staggered antennas,tracking thread HSC 30 stores the information in a FIFO structure 710,722 or 724 in association with a variable equal to the global TACvariable 222 at the time the RFID information is received, plus thenumber of TAC pulses that correspond to the distance by which thereceiving antenna has been displaced from the position it wouldotherwise have occupied in the RF tunnel as shown in FIG. 1 (i.e. from aposition in which the detection zones for the tunnel antennas coincide).When the global TAC variable thereafter equals a stored TAC variable,tracking thread 200 processes the RFID tag information associated withthe stored TAC variable according to the procedure as described abovewith respect to staggered bottom antenna 76 and FIGS. 32 and 33.Accordingly, the delay in considering the RFID tag informationcompensates for the offsets of the antennas from their positions inwhich the antennas share a common detection zone, and information isconsidered by the tracking thread as if all four antennas were disposedat their tunnel positions as in FIG. 1, with respect to line 49 (seeFIG. 1).

As discussed above, a given package typically carries RFID tags of oneclass. Although staggering antennas as set forth in the above-describedembodiment allows each staggered antenna to constantly attempt to readRFID tags located on a package, the antenna generally spends half itsoperating time querying for a class of tags that will not be found onthe package. In an alternative embodiment, and referring to FIGS. 36 and37, a companion antenna identical in structure and operation to eachstaggered antenna is added to the conveyor system so that each of thetwo antennas in each antenna pair continuously queries for a respectiveone of the two tag classes, while its duplicate antenna continuouslyqueries for the other. The antenna pair therefore performs approximatelytwice as many queries as a single antenna, thereby increasing efficiencyand accuracy.

The companion antennas are disposed together as a group as a replicationof the initial antenna group and are disposed upstream from the initialgroup. The dimensions and orientation of the companion antennas is thesame for the initial antennas, and components of the two antenna groupsare designated in FIG. 36 with “a” and “b” reference number suffixes.Each antenna is, again, offset sufficient distances from the otherantennas so the antennas do not interfere with each other. Respectiveengines 72 a, 52 a, 62 a, 80 a, 72 b, 52 b, 62 b and 80 b drive antennas66 a, 46 a, 56 a, 76 a, 66 b, 46 b, 56 b and 76 b and are controlled inturn by HSC 30. Engines 52 a, 80 a, 62 a, 72 a, 52 b, 80 b, 62 b and 72b are controlled by respective sequence threads 204 a, 706 a, 718 a, 720a, 204 b, 706 b, 718 b and 720 b and output received RFID information torespective read threads 716 a, 212 a, 718 a, 720 a, 716 b, 212 b, 718 band 720 b.

Each of the sequence generators defines a sequence that includes an idand corresponding instructions only for its respective antenna. Thecorresponding sequence thread constantly checks queue flag 224. If queueflag 224 indicates that a package structure is in package queue 226, theantenna sequence thread initializes its corresponding antenna engine andrequests a sequence from the antenna sequence generator. Because thesequence includes only the single corresponding antenna, the sequencethread continuously activates the antenna as long as a package structureis in the queue. The sequence generator defines the power level at whichto drive the antenna during its activation, the class of RFID tags toattempt to read during the antenna activation (for example, read class 0for the “a” antennas, and read class 1 for the “b” antennas), and thelength of time the antenna polls for a tag.

Each antenna offset upstream from its tunnel position functions in thesame manner as described above regarding bottom read antenna 76 in FIGS.32 and 33. When HSC 30 receives any RFID information from any of thestaggered antennas, tracking thread 200 stores the information in a FIFOstructure (710 a, 722 a, 724 a, 710 b, 722 b, 724 b and a FIFO structurefor staggered top antenna 46 a in communication with and between a readthread 716 a (not shown) and tracking thread 200) in association with avariable equal to the global TAC variable 222 at the time the RFIDinformation is received, plus the number of TAC pulses that correspondto the distance by which the receiving antenna has been displaced fromthe position it would otherwise have occupied in the RF tunnel as shownin FIG. 1. When the global TAC variable thereafter equals a stored TACvariable, tracking thread 200 processes the RFID tag informationassociated with the stored TAC variable according to the procedure asdescribed above with respect to staggered bottom antenna 76 and FIGS. 32and 33. Accordingly, the delay in considering the RFID tag informationcompensates for the offsets of the antennas from their aligned positionsin which the antennas share a common detection zone, and information isconsidered by the tracking thread as if all eight antennas were disposedat their respective tunnel positions in FIG. 1, with respect to line 49(see FIG. 1).

The number of staggered antennas along the conveyor system is generallyequal to a multiple of the number of antennas the conveyor system wouldemploy within a single RF antenna tunnel described with respect toFIG. 1. For example, where each antenna transmits and receives signalsfor only one class of RFID tags, the number of antennas is preferablyequal to the number of tag types the system is expected to handle (e.g.two when the system handles class 0 and class 1, or three where thesystem additionally processes Gen2 tags) multiplied by the number ofantennas employed by one RF antenna tunnel. This allows each antenna tocontinuously query for tags of a specific type.

A double row patch array may be configured so that it can be receivedand secured in the same frame, for example as shown in FIG. 7, as thesingle row patch array. Referring to FIG. 24, for example, patch arrayantenna 76 includes two rows of three patch elements 332 on the side ofa substrate 330 (see, e.g. FIG. 7) opposite the antenna's ground plane.Each patch is made of approximately 0.0014 inch thick copper or otherhigh-conductivity metal to form a corner-truncated square patch (patchsize of 5.25 inches per side before truncation and 4.4 inches per sideafter truncation) and is disposed in the substrate so that the top ofthe patch is flush with the top surface of the substrate. A corporatefeed network 338 drives patches 332, and the area required for the feednetwork is therefore reduced from the area that would be required ifboth rows of patch elements were fed by a dual point feed network asshown in FIG. 10. Accordingly, depending on the dimensions of thesubstrate of the single row array of FIG. 10 and the frame in which itis secured, the two rows and feed network of FIG. 24 may be mounted to asubstrate to fit in a frame of the same construction and dimensions asthe frame for the single row substrate.

A coaxial connector at 344 connects the antenna's printed circuit boardto an RF cable from engine 80 (FIG. 1). Feed network 338 is a corporatenetwork that combines the power received from each patch element anddelivers the combined signal to a center conductor along separate pathsfrom respective patch elements 332 a-332 f. Each path includes arespective single feed line 347 a-347 f attached mid-way along one sideof the patch element. Each feed line 347 a, 347 b, 347 c, 347 d, 347 eand 347 f is attached at its opposite end to a corner of a 3 dBquadrature hybrid coupler 350 a, 350 b, 350 c having sides of a length(approximately 2.8 inches in the presently described embodiment)approximately one-quarter wavelength of the signal carried by the feednetwork. As should be understood in this art, the antenna wavelength canbe affected by, for example, the antenna's substrate and cover materialpermittivity, and the signal specifics, as well as antenna dimensions,are provided herein for purposes of explanation and not in limitation ofthe present invention. Two sides of each quad coupler are compressed toallow disposition of the couplers between the two patch element rows. Atrace (not shown) extends from a first bottom corner 352 and isconnected to ground through a resistor. The opposite corner on the leftside of the quad coupler is connected to the feed trace.

The feed trace extending from quad coupler 350 has an impedance of 130ohms, whereas entry point 344 exhibits an impedance of approximately 150ohms to all three traces. Accordingly, a one-quarter wave element may bedisposed within the feed trace to match the impedances. Also in thisembodiment, patch array antenna 76 operates within a frequency range of902 to 928 MHz, and the feed network and patch elements are preferablyconstructed so that there is less than −15 dB return loss within thisfrequency range or otherwise within the frequency range in which theantenna is desired and/or designed to operate.

The patch elements are aligned in sequence in each row extendingtransverse (the X direction) to the path of the conveyor belt so thatcenter patches 332 b and 332 e are disposed in the center of the belt'spath. The patches in each row are preferably aligned in tandem, so thatthe side patches are aligned with the center patches in the transversedirection, and the distance from patch corner 366 to patch corner 368 isapproximately 25.4 inches in this embodiment, or approximately the widthof the conveyor belt that, in a typical retail distribution center, canbe expected to range in width between 24 and 28 inches. Thecenter-to-center spacing between adjacent patches is approximately 9inches.

In operation, and assuming the midpoint of the 902 MHz-928 MHz range inthis embodiment (i.e. 915 MHz), the center conductor applies the drivesignal to entry point 344 so that the drive signal is then applied byfeed traces 333 a-333 c to corners 335 a-335 c of the respective quadcouplers. Center feed trace 333 b is one-quarter wavelength longer thanfeed trace 333 a. Thus, the signal provided to center quad coupler 350 blags the signal at left coupler 350 a by 90°. Right trace 333 c isone-half wavelength longer than feed trace 333 a, and the signalprovided to right coupler 350 c lags the signal at left coupler 350 a by180°.

Because the length of each leg of each quad coupler is one-quarter ofthe drive signal wavelength, each leg introduces a one-quarter phaseshift in the signal. Thus, if the drive signal applied at the left quadcoupler is at 0°, the signal applied to patch 332 d (at its feed trace347 a) is at 270°, and the signal applied at patch element 332 a (at itsfeed trace 347 d) lags by an additional 90°, to 180°. As should beunderstood in this art, the differences in length in the feed traces, aswell as the differences in length of paths in the quadrature couplers,may result in slight differences in power applied to the patches, butthis affect may preferably be offset at least to some degree throughadjustment of trace width in the couplers.

If the drive signal applied at the left coupler is at 0°, theone-quarter wavelength increase in length from trace 333 a to trace 333b results in a signal at 270° applied at the top left corner of centercoupler 350 b. Thus, the signal applied to patch 332 b (at feed trace347 b) is at 180°, and the signal applied at patch element 332 e (atfeed trace 347 e) lags by an additional 90°, to 90°. As discussed inmore detail below, electric current in center patches 332 b and 332 erotates opposite to the direction of rotation of electric current inside patch elements 332 a, 332 c, 332 d and 332 f. If the length oftrace 333 b is equal to the length of trace 333 a, the total power andgain pattern from the antenna would be the same as that of the presentlydescribed embodiment, but the polarization of both gain pattern lobeswould be in the Y direction rather than the X and Y directions,respectively, as discussed below. Such a configuration is within thescope of the present invention, as are other patch and/or feed networkconfigurations, as should be apparent upon review of the presentdisclosure.

If the drive signal applied at the left quad coupler is at 0°, theone-half wavelength increase in length from trace 333 a to trace 333 cresults in a signal at 180° applied at the bottom left corner of rightsquare connector 350 c. Thus, the signal applied to patch 332 f is at90°, and the signal applied at patch element 332 c lags by an additional90°, to 0°.

As indicated in FIG. 24, opposing corners of each patch element 332 aretruncated. As should be understood in this art, where such a clippedpatch element is fed at a single point, the surface electric currentpattern in the patch element rotates in the direction from the feedpoint away from the adjacent clipped corner. Thus, the electric currentpatterns on patches 332 a, 332 c, 332 d and 332 f rotate in thecounterclockwise direction (in the perspective of FIG. 24), whereas theelectric current patterns on patches 332 b and 332 e rotate in theclockwise direction. Referring again specifically to patch elements 332a and 332 d, the patch elements are disposed in tandem with respect toeach other so that their corner-to-corner axes in the Y direction arealigned. With this orientation as a reference and as shown in FIG. 24,feed line 347 a attaches to a side of patch element 332 a that isrotationally offset 90° in the counterclockwise direction with respectto the side of its corresponding tandem patch element 332 d to whichfeed line 347 d attaches. Because the electric current patterns onpatches 332 a and 332 d rotate in the same (counterclockwise) direction,the 90° counterclockwise relative offset between the feed pointsintroduces a 90° lead in the electric current pattern in patch 332 awith respect to the electric current pattern on patch 332 d. Thiscompensates for the 90° lag in the signal applied to patch 332 a withrespect to the signal applied to the corresponding patch element 332 darising from leg 602 in quad coupler 350 a. Accordingly, the electriccurrent patterns in patches 332 a and 332 d are in phase with respect toeach other as the patterns rotate.

Patch elements 332 c and 332 f, including the clipped corners and feedpoints for feed lines 347 c and 347 f, are disposed in the sameorientation with respect to each other as are patch elements 332 a and332 d. Like feed line 333 a, feed line 333 c attaches to the lower leftcorner of the quad coupler. Thus, the electric current patterns forthese patches are also in phase with respect to each other as thepatterns rotate. Further, the tandem alignment of patches 332 c and 332f is parallel to the tandem alignment of patches 332 a and 332 d.However, because feed line 333 c introduces a half-wavelength lag in thesignal applied to the coupler for patches 332 c and 332 f with respectto the signal applied to the coupler for patches 332 a and 332 d, theelectric current patterns of patch elements 332 c and 332 f are 180° outof phase with respect to the electric current patterns of patch elements332 a and 332 d. The electric fields of the corresponding radiationpatterns of patches 332 a and 332 d are 180° out of phase with respectto the electric fields of the corresponding radiation patterns ofpatches 332 c and 332 f, at points equidistant from patches 332 a and332 d, on one hand, and patches 332 c and 332 f, on the other hand.

Center patch elements 332 b and 332 e are also disposed in tandem withrespect to each other so that their corner-to-corner axes in the Ydirection are aligned. The tandem alignment of patches 332 b and 332 eis parallel to the tandem alignment of patches 332 a/332 d and patches332 c/332 f, and feed lines 347 b and 347 e attach to patches 332 b and332 e at the same respective sides as feed lines 347 a/347 d and 347c/347 f attach to patches 332 a/332 d and 332 c/332 f. Again, the feedline attaches to a side of the upper patch element (332 b) that isrotationally offset 90° in the counterclockwise direction with respectto the side of the lower patch element (332 e) to which the feed lineattaches. However, because the electric current patterns on patches 332b and 332 e rotate in the clockwise direction, the 90° counterclockwiserelative offset between the feed points introduces a 90° lag in theelectric current pattern in patch 332 b with respect to the electriccurrent pattern on patch 332 e. This compensates for the 90° lead in thesignal applied to patch 332 b with respect to the signal applied topatch 332 e arising from leg 604 in quad coupler 350 b. Accordingly, theelectric current patterns in patches 332 b and 332 e, and therefore theelectric fields in the patches' radiation patterns at equidistantpoints, are in phase with respect to each other as the patterns rotate.

FIGS. 25A-25C illustrate the far field gain pattern resulting from theantenna configuration shown in FIG. 24 in perspective view, the X-Zplane and the X-Y plane, respectively. The dual-main lobe pattern issimilar to the pattern shown in FIGS. 11A and 11B for the single rowarray, but the pattern is narrower in the conveyor belt's direction oftravel, i.e. the Y direction. Accordingly, the dual row design increasesthe area of the antenna's near field pattern in the Y direction, therebyincreasing the likelihood the antenna will desirably read RFID tagspassing over the antenna on the underside of packages, while decreasingthe antenna's far field pattern in the Y direction, thereby reducing thelikelihood the antenna will undesirably read tags on other packages.Additional rows may further emphasize this affect and are encompassed bythe present disclosure. Preferably, the width of patch rows in suchantennas does not exceed the minimum Y direction detection zone widthexpected in the system.

Referring to FIG. 26A, the total gain pattern 606 (taken in the X-Zplane) is comprised of a linearly polarized first pattern 608 and alinearly polarized second pattern 610 that is offset 90° in polarizationwith respect to first pattern 608. FIGS. 26B-26D illustrate the samegain patterns, taken in the (45°X, 45°Y)-Z plane, Y-Z plane and (135°X,135°Y)-Z plane, respectively. The gain for the dual row patterndescribed herein is approximately 3 dB higher than the single newembodiment described with respect to FIG. 10. Accordingly, power to thedual row antenna is reduced to maintain the gain within FCC regulations,e.g. that the effective isotropic radiated power be not greater than 36dBm. For example, in the dual row embodiment described herein, the gainis greater than 6 dB, and the patches are therefore preferably driven ata power level less than 1 watt.

In another preferred embodiment of the present invention, and referringto FIGS. 27A-27G, interference antennas are provided upstream (withrespect to the belt's direction of travel 28) from the query/receiveantennas in the RF tunnel to radiate radio frequency signals thatinterfere with reception of and response to radio frequency querysignals from the query/receive antennas and thereby block responses fromtags upstream from the detection zone. The reduced likelihood thequery/receive antennas 46, 56, 66 and 76 (antenna 76 not shown; seeFIG. 1) will receive responses from upstream tags reduces the need forRF absorbers proximate the antennas, and RF absorbers are thereforeomitted in the illustrated embodiment. Otherwise, the construction andorientation of side antennas 56 and 66, top antenna 46 and bottomantenna 76 is the same as discussed above with respect to FIGS. 1,20A-20D and 24.

As described above, each of side antennas 56 and 66 is attached toantenna frame 44 (FIG. 1) so that the generally planar antenna isoriented in a vertical plane at a 45° angle with respect to a verticalplane including the conveyor belt's centerline and so that the antenna,and therefore the center of its radiation pattern, faces the conveyorbelt in the downstream direction at a 45° angle. In the embodiment shownin FIG. 1, this antenna orientation, in conjunction with the RF absorberpads, defines a reference line 71 (FIG. 20A) extending transverselyacross conveyor belt 14 at the upstream edges of query/receive antennas46, 56 and 66 and upstream of which the power level of signals from theradiation patterns emitted by the query/receive antennas is generallytoo low to activate RFID tags such that the RFID tags respond to theantennas. In the embodiments illustrated in FIGS. 27A-27G, however, thetransition between an area from which the query/receive antennas canreceive RFID tag transmissions, and from which they do not generallyreceive RFID tag transmissions, is defined by the presence ofinterference antennas 402, 404 and 416 proximate the query/receiveantennas.

More specifically, and referring initially to antenna 56, the antennaradiation pattern can be considered to extend from the antenna's frontface within an effective range extending approximately 60° to eitherside of lines 403 normal to the antenna's front face at the edges of thepatch array, as indicated schematically at 405. At 60°, the radiationpattern power level drops to approximately −3 dB and beyond 60° dropsrapidly further. As should be understood in this art, however, the pointat which the radiation pattern is sufficient to activate a tag such thatthe tag can effectively read and respond to the activating signal maydepend on factors such as the power at which the antennas are driven,the orientation of tags being read and the construction of the tagsthemselves. Thus, the effective range indicated at 405 can vary, and itshould be understood that the 60° spread is provided for purposes ofexplanation and not in limitation of the present invention.

The construction of interference antenna 402 is the same as that ofantenna 56 and is therefore not discussed further. Interference antenna402 is attached to the antenna frame so that the interference antenna isoriented in a vertical plane at a 60° angle with respect to a verticalplane 401 including the conveyor belt's centerline and so that theantenna, and therefore the center of its radiation pattern, faces theconveyor belt in the upstream directions at a 30° angle 399 with respectto centerline plane 401. Interference antenna 402 may be driven at thesame power level as antenna 56, and the antenna's radiation pattern(gain pattern) can be considered to extend from the antenna's facewithin an effective range extending approximately 60° to either side oflines 407 normal to the antenna's front face at the edges of itspatches, as indicated schematically at 409 and 411.

The power at which the interference antennas are driven is notnecessarily the same as the power their adjacent query/receive antennas46, 56 and 66 are driven, but this can be used as a starting point in atesting procedure to determine the appropriate power. More specifically,packages on which RFID tags are disposed (or, simply, the tags) areplaced on the belt in various positions upstream from line 411. Power tothe interference antennas is then scaled up and down until a level isreached at which the operating antennas do not read the RFID tags or doso at an acceptable rate. The packages (or tags) are then disposed atvarious positions downstream from line 49 so that the RFID tags arewithin an expected detection zone, and power is applied to thequery/receive antennas according to an antenna sequence as describedabove while power is supplied to the interference antennas at the leveldetermined in the first stage of the test. If the query/receive antennasread the tags without interference from the interference antennas, theinterference antenna power level may be acceptable, and further testscan be performed based on packages carried by the belt when in motion.If, however, the interference antennas undesirably interfere with theability of the query/receive antennas to read RFID tags, then the powersupplied to the interference antennas is reduced until an acceptableread rate is achieved. The first and second steps are repeated until aninterference power level is defined that both prevents reads upstreamfrom the detection zone and allows reads within the detection zone atrates acceptable or desirable within the context of the particularsystem. In an alternate method, the gap between lines 411 and 49 iseliminated, and RFID tags are placed upstream and downstream from line411 (or other line at another desired distance upstream from thedetection zone) in the first and second test stages, respectively. Whilethis second method may require acceptance of some degree of interferencedownstream from the transition line, and some degree of tag readsupstream from the line, it may provide a more precisely-definedtransition.

It may also be desirable to drive the interference antenna at powerlevels lower than the query/receive antenna power levels to reduceoverall in-band noise. In-band noise can be more or less problematic ina given RF system, for example depending on surfaces proximate to the RFantennas that may reflect noise to the antennas. Where relativelygreater RF reflective surfaces are present proximate the antennas, itmay be desirable to reduce the interference antenna power. Furthermore,it should also be understood that it is not necessary that theinterference antennas be constructed similarly to the query/receiveantennas, provided the interference antennas reduce undesired upstreamRFID tag responses to the degree desired for a given RF system.

As described in more detail below, the transmission from interferenceantenna 402 reduces the likelihood that an RFID tag receiving theinterference antenna's signal will respond to a signal also received byantenna 56 or any of the other query/receive antennas. Thus, even thoughthe radiation pattern from antenna 56 extends upstream from line 411,the presence of the radiation pattern from interference antenna 402generally prevents undesired responses from RF tags upstream from line411.

Moreover, interference antenna 402 is disposed with respect to thequery/receive antenna and the belt so that antenna 402 radiates radiofrequency signals into an area that is proximate to and generallyexcludes the common detection zone defined for the query/receiveantennas. The signals from antenna 402 can be considered to be entirelyexcluded from the detection zone when the signals from antenna 402within the detection zone are such that they would not prevent a radiofrequency tag from responding to query signals from the query/receiveantenna anywhere within the detection zone. This may occur, for example,if the power level of the signals from antenna 402 is such that a radiofrequency tag in the detection zone would not distinguish the signalsfrom antenna 402 from noise, or if the power level of the signals fromantenna 402 within the detection zone is otherwise sufficiently belowthe query/receive antenna query signal power level that the tag alwaysresponds to the query signal. The signals from antenna 402 may beconsidered to be generally excluded from the detection zone if the gainpattern from antenna 402 extends into the detection zone defined by thestart window and stop-read window, and interferes with the reception ofand response to query signals from the query/receive antennas by RFIDtags in the detection zone, but does so to less than the full extent ofthe detection zone and leaves a portion of the detection zone in whichthe signals from antenna 402 do not interfere with the query/receivequery signals that is sufficiently large that the system achieves a rateof successful reads of radio frequency identification tags on packagespassing through the detection zone that is acceptable under theconditions and criteria under which the system is designed to operate.Thus, the signals from antenna 402 may interferingly extend into themajority of the detection zone, provided the remaining part of thedetection zone is sufficient to permit the query/receive antennas toread radio frequency identification tags at an acceptable rate.

Because the effective antenna radiation patterns may vary, line 411 isnot precise, and it is possible that RFID tags slightly upstream fromline 411 may receive and respond to signals from the query/receiveantennas (46, 56, 66 and 76) and/or that signals from the interferenceantennas may prevent RFID tags slightly downstream from line 411 fromresponding to the query/receive antennas. The area upstream anddownstream from line 411 in which such conditions may occur can dependon the particular antenna construction and power but, in the presentlydescribed embodiment, does not extend into the detection zone downstreamfrom line 49. In another embodiment illustrated in FIG. 27B,interference antenna 402 is attached to the antenna frame so that theinterference antenna is oriented in a vertical plane at 45° angle 399with respect to a vertical plane including the conveyor belt'scenterline and so that the antenna, and therefore the center of itsradiation pattern, faces the conveyor belt in the upstream direction ata 45° angle. A 45° orientation of antenna 402 can be preferred topotentially increase the number of RFID tags for which the antenna caneffectively interfere. While the resulting line 411 extends across belt14 at a 15° angle with respect to a line 413 normal to the belt's movingdirection, line 411 does not cross into the detection zone or does soonly minimally.

Returning to FIG. 27A, the construction and orientation of interferenceantenna 404 is the mirror image of the construction and orientation ofinterference antenna 402. Interference antenna 404 is, preferably,driven at the same power level as antenna 402, and the antenna'sradiation pattern can be considered to extend from the antenna's facewithin an effective range extending approximately 60° to either side ofthe antenna's front face, as indicated at 419 and 411. Referring also toFIG. 27C, top interference antenna 416 is disposed on the antenna frameso that its downstream edge is aligned with line 411 (expanded as aplane in FIG. 27C) and so that the interference antenna spanstransversely across the path of belt 14 at a 60° angle in the upstreamdirection with respect to a horizontal plane parallel to the conveyorbelt. Interference antenna 416, and therefore the center of itsradiation pattern, faces the conveyor belt in the upstream direction ata 30° angle 415. The interference antenna's radiation pattern can beconsidered to extend from the antenna's face within an effective rangeextending approximately 60° to either side of the antenna's front face.That is, the downstream end of the effective radiation pattern frominterference antenna 416 extends generally vertically at plane 411.Accordingly, the three interference antennas define a generally verticalboundary at plane 411 upstream of which RFID tags should not respond tosignals from antennas downstream from plane 411.

Preferably, the power level at which top interference antenna 416 isdriven is higher than the power of antennas 402 and 404. Because topantenna 416 is disposed farther from the both than antennas 402 and 404,a higher power level may be needed to create a gain pattern sufficientto interfere with reception and response of RF signals by RFID tagsproximate the belt. As a result, antenna 416 may create interferencedownstream of line 411 to an extent greater than antennas 402 and 404,although one skilled in the art should recognize that this effect can beconsidered when setting the antenna's power level as described above.

As noted with respect to interference antenna 402, the orientation ofantennas 404 and 416 may vary. In the preferred embodiment in whichantenna 402 is disposed at a 45° angle with respect to vertical plane401, interference antenna 404 is also disposed at a 45° angle withrespect to the vertical plane and so that the antenna, and therefore thecenter of its radiation pattern, faces the conveyor belt in the upstreamdirection at a 45° angle. Referring also to FIG. 27D, top interferenceantenna 416 is disposed on the antenna frame so that its downstream edgeis aligned with the downstream edges of antennas 402 and 404 (as shownin FIG. 27A) and so that the interference antenna spans transverselyacross the path of belt 14 at a 45° angle in the upstream direction withrespect to a horizontal plane parallel to the conveyor belt. As withantenna 402, the effective interference radiation patterns extend fromantennas 404 and 416 at an approximately 15° angle downstream from avertical plane 417 extending transverse to the belt's path of travel andaligned with the interference antennas' downstream edges but preferablydo not cross into, or cross into only minimally, the detection zonedownstream of line 49.

Radiation patterns from the three interference antennas preferably coveran area sufficiently far upstream from plane 411 to prevent RFID tagsthat are otherwise close enough to the antenna tunnel to successfullytransmit to the query/receive antennas from receiving and responding tosignals from any of the query/receive antennas either directly or asreflected from RF-reflective surfaces proximate the tunnel. In thepresently described embodiment, no interference antenna is providedproximate bottom antenna 76 (FIG. 1). The three interference antennascollectively generate a radiation pattern that can interfere with RFIDtags on any surface of the package, regardless to which of thequery/receive antennas the tag might otherwise respond. Depending, forexample, on a package's contents it is possible for RFID tags on apackage's bottom surface to respond only to bottom antenna 76 and not tobe affected by the side and top interference antennas. The bottomantenna's radiation pattern is, however, relatively narrow in the areaimmediately above the belt's surface in which these tags are located.Accordingly, there is less risk the bottom antenna will undesirablyactivate and receive a signal from such a tag upstream from thedetection zone, and a bottom interference antenna, while within thescope of the present disclosure, is therefore omitted in the presentlydescribed embodiments.

In another preferred embodiment of the present invention, and referringto FIGS. 27E and 27F, RF absorber pads are disposed between respectivepairs of query/receive and interference antennas to establish a moredefinitive transition between an area from which the query/receiveantennas can generally receive RFID tag transmissions and an area fromwhich interference antennas 402, 404 and 416 generally prevent suchtransmissions. As described above, the power level of the radiationpatterns emitted by antennas 46, 56 and 66 is generally too low toactivate RFID tags upstream from line 71 such that the tags respond tothe transmitting antennas, due to the orientation of antennas 46, 56 and66 and the presence of RF absorber pads 48, 58 and 68. While the signalpower increases between line 71 and the start of the detection zone atline 49, the power level in this area preferably remains generallyinsufficient to so activate an RFID tag, although it should beunderstood that the point at which the radiation pattern activates a tagmay depend on factors such as the power at which the antennas aredriven, the orientation of tags being read, the RFID tags' configurationand the antenna orientation. For purposes of this discussion, however, aline 400 is assumed to define the point at which the radiation patternsfrom antennas 56 and 66 become sufficiently strong to activate a tagdisposed on an outer surface of a package 24 traveling on the conveyorso that the tag respond to the transmitting antenna.

The construction of the interference antennas is the same as in theprior embodiment. Interference antenna 402 is disposed beside conveyorbelt 14 at a 45° angle with respect to the belt's centerline andapproximately two inches above the conveyor frame. A metal plate 406extends vertically upward from the conveyor and has an L-shapedcross-section in the X-Y plane. The length of each leg of metal plate406 is approximately 7.5 inches. The length (in the Y direction) offerrite absorber pads 408 is approximately 7.75 inches, such that theabsorber pads extend slightly upstream of the upstream edge of metalplate 406. Absorber pads 408 are disposed on both sides of the metalplate—i.e. on the side facing the antenna and the side facing the belt.

The construction and orientation of interference antenna 404 is themirror image of the construction and orientation of interference antenna402. A reference 410 can be considered to extend transversely acrossconveyor belt 14 at the back edges of antennas 402 and 404. Due to theorientation of antennas 402 and 404 and the presence of RF absorber pads408 and 412, the power level of any signal from the radiation patternsemitted by interference antennas 402 and 404 is too low to effectivelyinterfere with reception of and response to RF signals by RFID tagsdownstream from line 410. As should be understood by those skilled inthis art, the point upstream from line 410 at which the radiationpattern effectively interferes with a tag may depend on factors such asthe power at which the antennas are driven, the orientation andconfiguration of the RFID tags, and the antenna orientation. Forpurposes of this discussion, however, a line 414 is assumed to definethe point at which the radiation patterns from interference antennas 402and 404 become sufficiently strong to effectively interfere with a tagdisposed on the side of a package 24 traveling on the conveyor.

Referring to FIG. 27F, top interference antenna 416 is disposed so thatits downstream edge is aligned with reference line 410 and at a 45°angle with respect to a plane parallel to the conveyor belt. Absorberpads 48 extend slightly upstream from reference line 410.

In a still further embodiment, the side absorber pads (and theirsupporting metal plates) are omitted, and absorber pad 48 is shortenedso that its upstream edge is at the downstream edge of interferenceantenna 416 or between the downstream edge of antenna 416 and theupstream edge of antenna 46. Accordingly, the interference antennas maybe driven at power levels lower than that when the absorber pads arepresent, and the interference antennas may interfere with undesiredtransmissions from RFID tags between lines 410 and 414 that might resultfrom reflected transmissions originating from the query/receiveantennas.

In each of the embodiments shown in FIGS. 27A-27F, interference antennas402, 404 and 416 are connected by feed lines 418, 420 and 422 torespective antenna engines 424, 426 and 428 that drive transmissionsignals to the antennas. Because the interference antennas do notreceive responses from the RFID tags, there is no need for the enginesto receive and process return signals, and in another preferredembodiment, engines 424, 426 and 428 are replaced by transmitters thatdrive the antennas to transmit interference signals but do not receivesignals from the antennas. Alternatively, a single engine or singletransmitter supporting three or more output ports, or having a singleoutput port to an antenna switch controlled by HSC 30, can be used todrive all three interference antennas, e.g. in a round-robin sequence.

Referring to FIG. 27A, respective output lines 430, 432 and 434 connectantenna engines (or transmitters) 424, 426 and 428 to HSC 30, whichcontrols the engines to drive the antennas. Because interferenceantennas 402, 404 and 416 operate simultaneously in the presentlydescribed embodiment, cables between HSC 30 and the antenna engines arepreferably of the same length, as are the cables between the engines andthe interference antennas, so that the signals from the interferenceantennas are in phase and do not cancel. Each engine includes atransmitter and a microprocessor that controls the transmitter andcommunicates with HSC 30 over the engine's output line. HSC 30 initiatesa transmission from the antenna by a command to the enginemicroprocessor. In response, the microprocessor sends a bit sequence tothe transmitter, which then transmits the signal at a specifiedfrequency and power level to the antenna.

Referring also to FIG. 28, antenna sequence thread 204 controlsinterference RF engines 424, 426 and 428 and is controlled in turn byantenna sequence generator 202. In the presently described embodiments,antenna sequence generator 202 defines the order in which the antennasequence thread is to activate the transmit/receive antennas, forexample in round-robin fashion. In one preferred embodiment, sequencegenerator 202 instructs antenna sequence thread 204 to simultaneouslyactivate interference antennas 402, 404 and 416 whenever any of thequery/receive antennas are activated. Thus, for example, assume id's 1,2, 3 and 4 respectively refer to antennas 56, 66, 46 and 76, and thatid's 5, 6 and 7 respectively refer to interference antennas 402, 404 and416. If the sequence generator defines a round-robin sequence among allfour transmit/receive antennas, the sequence provided by sequencegenerator 202 to sequence thread 204 is (a) 1/5/6/7, (b) 2/5/6/7, (c)3/5/6/7 and (d) 4/5/6/7. Upon reading the first step (a) in thesequence, for example, antenna sequence thread 204 simultaneouslyinstructs engines 62, 424, 426 and 428 to drive their respectiveantennas 56, 402, 404 and 416. At step (b), the antenna sequence threadsimultaneously instructs engines 72, 424, 426 and 428 through therespective read threads to drive their respective antennas 66, 402, 404and 416, and so on. Accordingly, interference antennas 424, 426 and 428are activated until antenna sequence thread 204 detects from queue flag224 that no packages are present, at which point the sequence threadstops driving the antennas through their respective engines. Readthreads 222 are provided for transmit/receive antennas 46, 56, 66 and 76and are preferably provided for the interference antennas forcommunication between the sequence thread and the interface antennaengines, although the system does not read response signals from theinterference antennas. In a still further embodiment, a separatesequence generator/sequence thread is provided for each interferenceantenna (in an arrangement similar to the sequence thread arrangementsshown in FIGS. 33, 35 and 37), while sequence generator/sequence thread202/204 drives the query/receive antennas as discussed above.Preferably, the interference antennas are activated only when there isat least one package in the package queue (i.e. when the queue flagindicates to the interference antenna's sequence thread that a packageis present). Under this condition, the respective sequence generatorsinstruct the interference antenna sequence threads to continuouslyactivate the interference antennas. In one embodiment, the interferenceantenna sequence threads do not communicate with stored information at240.

In an alternate embodiment, and referring to FIG. 27G, the interferenceantennas are activated only when there is at least one package in thepackage queue but also only if at least one such package is within apredefined area 432 upstream from start line 49 in which there exists alikelihood that an RFID tag on the package can transmit a response thatcould be received by the query/receive antennas in the RF antennatunnel. The length 434 of area 432 upstream from line 49 can depend atleast in part on system configuration and ambient conditions. Forexample, length 434 may be greater for a system in which the tunnelquery/receive antennas transmit at a relatively high power level and aresurrounded by RF reflective surfaces than for a system in whichquery/receive antennas transmit at a lower power level and aresurrounded by fewer RF reflective surfaces. If the system is operatedover periods in which packages are not present in area 432, theresulting reduction in operation of the interference antennas can reducein-band noise.

In this embodiment, referring also to FIG. 28, upon creation of apackage structure 228 for a package entering the system, tracking thread200 establishes an interference start window distance representing theTAC pulse distance between photodetector 36 (FIG. 1) and area 432. Ateach incoming TAC pulse, the tracking thread decrements the interferencestart window so that, at any given time between the point at which thepackage's leading edge enters the photodetector 36 (FIG. 1) line ofsight and the point at which the package's leading edge reaches area432, the interference distance variable represents the number of TACpulses remaining between the package's leading edge and area 432. Thatis, the interference start window distance represents the distance thepackage needs to travel before entering area 432.

Package structure 228 also includes an interference stop window distancevariable that defines length 434 of area 432. When a package firstenters the tracking system at photodetector 36 (FIG. 1), tracking thread200 initializes the interference stop window distance to a predefineddistance in TAC pulses. When the interference start window distance fora given package structure decrements to zero, tracking thread 200 beginsdecrementing the package structure's interference stop window distancevariable at each incoming TAC pulse. Tracking thread 200 also constantlychecks package structures 228 in package queue 226. If there exists anypackage structure having an interference start window that hasdecremented to zero and a non-zero interference stop window distancevariable, a package is present in area 432, and tracking thread 200notifies antenna sequence generator 202 to output an antenna sequenceincluding the interference antennas, as described above or, ifindividual sequence generators/sequence threads are provided for theinterference antenna engines, tracking thread 200 notifies the threeinterference antenna sequence generators to output antenna sequences totheir respective sequence threads to drive their respective interferenceantennas. If there is no package structure with a zero interferencestart window distance and a non-zero interference stop window distance,no package is present in area 432, and tracking thread 200 instructssequence generator 202 to output an antenna sequence without theinterference antennas.

For each antenna in the sequence, including the interference antennas,sequence generator 202 defines the power level at which the antenna isdriven during its activation and the length of time the antenna isactivated. Preferably, the three interference antennas are activatedover at least the same time period during which the query/receiveantennas are activated.

While respective engines 52, 62, 72 and 80 may be employed to driveantennas 46, 56, 66 and 76, in one preferred embodiment a single enginedrives the four query/receive antennas through an RF switch, asdiscussed above with respect to FIG. 32. In this embodiment, theinterference antennas are preferably driven by respective engines wherethe interference antennas are driven simultaneously with each other andthe query/receive antennas.

In the presently described embodiments, engines 52, 62, 72 and 80 drivethe query/receive antennas at a carrier signal that frequency hopswithin a 902-928 MHz range and amplitude shift key modulate the carrierto define the data on the signal according to the various RFID tagprotocols. HSC 30 defines the power at which the engine drives theantenna, while the engine defines the dwell time between frequency hops,the data rate (i.e. the rate at which the engine modulates the carrierto put data on the carrier signal) and the timing of the signal's databits.

While the class 0, class 1 and Gen2 protocols differ, each is configuredto operate with a frequency hopping transmitter. Class 0 begins with anumber of timing bits. Assuming class 0, upon detecting an incomingquery signal, an RFID tag begins a sequence based on the timing bitsthat sets the RFID tag's receiver timing so that the tag can receive theremainder of the message on the query signal as dictated by the tag'sprotocol. If, however, the tag simultaneously receives an interferencesignal that prevents the tag from setting its timing and/or receivingthe subsequent instruction, the RFID tag does not respond to the querysignal.

In one preferred embodiment, for example, engines 424, 426 and 428 drivetheir respective interference engines at a sine or square-wave signal ata frequency specific to the protocol at which the query/receive antennais transmitting. Where the data rate and bit timing are unknown, aninterference signal frequency can be determined through testing. Aquery/receive antenna is disposed coplanar with an interference antennaso that the antennas face in the same direction. An RF tag is disposedbetween and in front of the antennas a distance sufficient (in oneexample, approximately 8.25 inches) for the tag to receive signalsequally from the two antennas. In a given test, the query/receiveantenna transmits a query signal at a known power level and according tothe tag's protocol. Simultaneously, the interference antenna is drivenat the same power level by a square wave generator at a selectablefrequency and depth, and the output of the query/receive antenna ismonitored to determine if the tag responds to the query signal. “Depth”refers to the modulation depth, or the ratio of maximum carrieramplitude to the minimum carrier amplitude, in the square wave pattern.For example, a 100% depth indicates the signal changes from full carrierto zero carrier according to the square pattern, whereas a 50% depthindicates the carrier varies between full and half according to thesquare pattern. A test comprises a selected number (e.g. 100) querysignals and corresponding attempts to read the RFID tag, and the test'sread percentage is the percentage of such attempts at which thequery/receive antenna successfully detected a response from the RFIDtag.

The table below describes the results of such a series of tests. Fivetests at 100 attempts per test were performed for each selectedtransmission frequency and depth. The interference and query/receiveantennas were three-element patch arrays as described herein. Thequery/receive antenna frequency hopped over the 902-928 MHz range. Theinterference antenna transmitted square waves at the various modulationdepths in the table below at a 915 MHz carrier frequency that did notfrequency hop. The data rate and bit timing of the RF engine driving thetransmit/receive antenna were unknown.

Interference Signal Read Percentage Freq. Depth Test 1 Test 2 Test 3Test 4 Test 5 No Signal No Signal 100 100 100 98 96 1 KHz 100% 27 58 2749 49 400 Hz 100% 29 26 45 31 33 20 kHz 100% 35 16 20 27 29 30 kHz 100%32 45 33 38 46 100 Hz 100% 62 59 83 62 51 10 kHz 100% 13 29 12 8 14 5kHz 100% 38 24 25 18 16 10 kHz  50% 57 62 39 51 81

As indicated in the table, a square wave at 915 MHz carrier and that isamplitude shift key modulated at 10 kHz with 100% depth produced themost effective interference of the signals from the transmit/receiveantenna. Referring again to the above-described embodiments, sequencegenerator 202 defines a sequence that triggers antenna sequence thread204 to instruct engines 424, 426 and 428 to drive antennas 402, 404 and416 (at a power level determined as described above) during the timeperiods when any of engines 52, 62, 72 and 80 activate the query/receiveantennas to query for a class 0 RF tag. The microprocessors at engines424, 426 and 428 are programmed so that, upon receiving instruction fromHSC 30/antenna sequence thread 204 to drive the interference antennas atthe class 0 power level, engines 424, 426 and 428 drive the respectiveinterference antennas at a power level determined as described abovewith a 915 MHz non-frequency-hopping signal that carries a square wavethat is amplitude modulated at 10 kHz with 100% depth. The carrierfrequency may frequency-hop where required by FCC regulations.

The same procedure may be used to determine the frequency of theinterference signal for each of the class 1 and Gen2 protocols. Thus,upon receiving instruction from HSC 30/antenna sequence thread 204,engines 424, 426 and 428 drive the respective interference antennas at apower level defined by the sequence thread with a 915 MHz signalcarrying a square wave that is amplitude modulated at the frequency anddepth determined by the test.

If the data rate and bit timing of the signals at which engines 52, 62,72 and 80 drive the query/receive antennas are known, the interferencesignals by which engines 424, 426 and 428 drive the interferenceantennas can be defined to interfere with specific aspects of the querysignals from the query/receive antennas. For example, the interferencesignal can be timed so that square wave pulses occur between the querysignals's timing bits, thereby preventing the RFID tag from setting itsinternal timing, or otherwise at a time according to the tag protocolthat prevents the RFID tag from responding to the query. Such anarrangement may be effected, for example, by synchronizing the data rateamong the antennas so that the interference bits can be timed as desiredto prevent RFID tag responses. The protocol of the interference signalis preferably specific to the tag protocol so that there is a likelihoodof signal interference.

In a further embodiment illustrated in FIGS. 29-31D, interferenceantennas interfere with undesired RF tag responses through cancellationof electric field (E-field) components in the radiation patterns of thequery/receive antennas rather than by interference with data carried bythe query/receive antennas' query signals. Antennas 46, 56, 66 and 76are constructed as described above with respect to FIGS. 1-21, exceptthat the RF absorber pads and their supporting frames are omitted. Sideand top antennas 56, 66 and 46 are patch arrays constructed as describedabove with respect to FIG. 8, while bottom antenna 76 (not shown; seeFIG. 1) is constructed as discussed with respect to FIG. 10.

Each of interference antennas 402 and 404 is attached to the antennaframe so that the interference antenna is oriented in a vertical planeat a 45° angle with respect to a vertical plane including the conveyorbelt's centerline and so that the antenna, and therefore the center ofits radiation pattern, faces the conveyor belt in the upstream directionat an acute angle, preferably 45°. More specifically, in the presentlydescribed embodiment, the top surfaces of the antenna patch elementsgenerally define a plane that intersects a vertical plane that includesthe belt's centerline in the area proximate the antenna. Topinterference antenna 416 is disposed on the antenna frame so that itsdownstream edge is aligned with the downstream edges of antennas 402 and404 and so that the interference antenna spans transversely across thepath of belt 14 and its generally planar front face extends in theupstream direction at a 45° angle with respect to a horizontal planeparallel to the conveyor belt.

FIG. 29 illustrates the arrangement of side antenna 56 and interferenceantenna 402 in relation to RF engine 62. The arrangement of each pair ofinterference and query/receive antennas (i.e. 402/56, 404/66 and 416/46)with respect to the engine driving the antenna pair is the same. Thus,while the present discussion is provided with respect to antennas 56 and402 and engine 62, it should be understood that the discussion appliesto the opposite side antenna pair and the top antenna pair and theircorresponding RF engines.

Engine 62 includes a transmitter that drives antennas 56 and 402 from anoutput port 500 and feed line 502 to a power splitter 504. The splitsignal from power splitter 504 is output to respective ports 506 a and506 b of three-port RF circulators 508 and 510, each of which defines aclockwise signal circulation from the perspective shown in FIG. 29. Port512 a of circulator 508 outputs to antenna 402, whereas port 512 b ofcirculator 510 outputs to antenna 56. Circulator 508 port 514 a outputsto a fifty ohm dead-end RF load 516 that is isolated from antennas 56and 402 and engine 62. Circulator 510 port 514 b outputs to an engineport 518 by a feed line 520.

As should be understood in this art, the signal provided by powersplitter 504 to circulator ports 506 a and 506 b is output to antennas402 and 56 by ports 512 a and 512 b, respectively. So that the signalsoutput by the two antennas are in-phase, the feed lines between ports512 a and 512 b and the respective antennas are of the same length.Signals received by the antennas are received by the circulators atports 512 a and 512 b and output at ports 514 a and 514 b, respectively,without passing to ports 506 a or 506 b. Thus, any signal received byinterference antenna 402 is directed to load 516 and therefore ignored,while signals received by query/receive antenna 56 is directed to thereceiver of engine 62 at port 518.

For purposes of explanation, FIGS. 31A-31D illustrate interferenceantenna 402 and query/receive antenna 56 oriented side-by-side so thattheir front faces are coplanar. Respectively corresponding FIGS. 30A-30Dschematically illustrate top views of the antennas in their actual 45°orientation to the belt (90° with respect to each other). Feed lines 346and 348 are applied to the patches on opposite sides in the twoantennas, as indicated in FIGS. 31A-31D. Because the signals applied tothe antennas are in phase, the direction of the electric current at thepatch surfaces of antenna 402 (indicated at 522) is opposite to thedirection of electric current at the patch surfaces of antenna 56(indicated at 524). As noted with respect to FIG. 8, the drive signalapplied to each patch element 332 by feed line 346 leads the signalapplied by feed line 348 by 90°, causing the electric current patternsin the patches to rotate in the counterclockwise direction, as indicatedat 526. The electric current at all patches of both antennas 56 and 402rotates in the same direction about respective axes perpendicular to thegenerally planar patch surfaces. As indicated in the sequence shown inFIGS. 31A-31D, electric current directions 522 and 524 oppose each otherthroughout the clockwise rotation.

The patches' electric current flow rotational direction determines thedirection of the electric field generated by the antennas. Referring toFIG. 30A, the E-field vector 528 for the radiation pattern of antenna402 can be considered as its component vectors 528 a and 528 b, whereasthe E-field vector 530 for the radiation pattern of antenna 56 can beconsidered as its component vectors 530 a and 530 b. As the antennas'electric current pattern rotates, so do the E-field vectors of thecorresponding radiation patterns, as shown in FIGS. 30B-30D. Asillustrated in the figurees, the component 528 b of E-field vector 528parallel to the direction of travel of conveyor belt 14 (i.e. the Ydirection) is always generally opposite the Y-direction component 530 bof E-field vector 530. The component 528 c of E-field vector 528 in thevertical direction (i.e. in the Z direction) is always generallyopposite the Z-direction component of E-field vector 530. However, thecomponent 528 a of E-field vector 528 transverse to the direction oftravel of conveyor belt 14 (i.e. the X direction) is always generallyaligned with the X-direction component 530 a of E-field vector 530.

Antennas 402 and 56 radiate the same signal at the same power.Accordingly, at a vertical plane 532 equidistant from antennas 402 and56, at which the radiation patterns from the two antenna patterns aretherefore at equal power, the Y and Z components of the two antennaradiation patterns generally cancel, while the X components add.Assuming a package on conveyor belt 14 is oriented square to the beltsuch that the package's front, and back sides are parallel to plane 532,only tags on the package front, back and top sides will generallyrespond to an E-field in the X direction. Tags on the package's bottomsurface could also respond, although they are typically too low torespond to the side antenna signal. Thus, at plane 532, only tags on thefront, back and top package sides should respond to the query signalfrom antennas 402 and 56 at plane 532. The radiation pattern emitted bythe package's front and back tags is generally centered along the Yaxis, with the pattern's strength weakening as it spreads in the Xdirections. The radiation pattern emitted by the top tags is generallycentered along the Z axis, with a component of the pattern's strengthweakening as it spreads in the X directions. The signal power reductionat an approximately 30° cone 534 from the front or back tag's radiationpattern centerline 536 is approximately 3 dB and reduces rapidly outsidethe cone. The signals from the top tags are similarly weak beyond 30° inthe X directions. Thus, the strength at the antenna patches of antenna56 of the radiation pattern from a front, back or top RFID tag at plane532 is generally insufficient to generate a response recognized byengine 62. That is, the radiation pattern signal is generallyindistinguishable from noise. Accordingly, and because cancellation ofthe query signal Y and Z E-field components prevents responses of RFtags on the package's side, surfaces, antenna 402 should prevent antenna56 from receiving a response from any RFID tag on the package when thetag is at plane 532.

Moving in the upstream Y direction from plane 532, the radiation patternfrom antenna 402 increases in strength over the pattern from antenna 56,and the likelihood increases that the Y and Z E-field components fromantenna 402 will activate an RFID tag on the side, top and bottomsurfaces of the package, despite the presence of counteracting E-fieldcomponents from antenna 56. The converse occurs moving in the downstreamY direction from plane 532, and the likelihood increases that the Y andZ E-field components from antenna 56 will activate an RFID tag on theside, top and bottom surfaces of the package, despite the presence ofcounteracting E-field components from antenna 402. The distance fromplane 532 in the downstream Y direction at which the E-field componentsfrom antenna 56 become sufficiently strong in comparison to thecorresponding components from antenna 402, such that an RFID tag on thepackage's side, top or bottom surfaces will be energized by andrecognizably respond to the radiation pattern, depends upon theconfiguration of the RFID tag, the power level and frequency of theradiation patterns from antennas 56 and 402, receiver sensitivity andthe orientation of antennas 56 and 402 with respect to each other andthe RFID tags, and the particular distance will therefore depend uponthe configuration in a given system. Preferably, this distance does notextend beyond line 49, the start of the detection zone. In one presentlypreferred embodiment having an arrangement as described herein in whichantennas 402 and 56 are each driven at a power level not greater than 1watt with a 915 MHz carrier signal, the Y and Z E-field radiationcomponents from antennas 402 and 56 sufficiently cancel so that RFIDtags on the side package surfaces within approximately two to threeinches upstream from plane 532, and approximately two to three inchesdownstream from plane 532, do not respond to the query signal.

Antenna 56 begins to receive signals from side RFID tags when the tagsmove downstream beyond the resulting no-read zone for those tags. Theantenna begins to read RFID tags on the top, front and back of thepackage when those tags move sufficiently downstream so that the 30°cone of the radiation pattern extending upstream from the tagencompasses antenna 56. Thus, while the no-read zones for the two groupsof tags are different, they overlap, and there is therefore an areaextending from either side of plane 532 in which the RF tags shouldeither not radiate an RF response signal or should not radiate an RFsignal that is detectable by antenna 56. It will be recognized thatbecause the no-read zones end upstream from line 49, antenna 56 may readtag responses before the package enters the detection zone. Such readsare assigned to deferred collection 238 (FIG. 2) if no package ispresently in the detection zone. If a package is in the detection zone,the algorithm described above should prevent application of the taginformation to such package, depending on the tag information otherwiseapplied to it.

The operation of side antennas 66 and 404, and the resulting no-readzones for RFID tags on the side, top and bottom package surfaces and onthe front and back package surfaces, is the same as for antennas 56 and402 and is therefore not further described. The operation of topantennas 46 and 416 is also the same as for antennas 56 and 402, butbecause these antennas face downward, instead of to the side, the X andY E-field components cancel, while the Z E-field component adds.

In a still further embodiment, illustrated in FIGS. 38 and 39, all fourquery/receive antennas 46, 56, 66 and 76 are disposed together at aneven position with respect to the belt's path of travel in the antennatunnel (e.g. as shown in FIG. 1). That is, the detection zones forrespective antennas coincide such that the tracking algorithm tracks thepackages (as described above) with respect to a single, common detectionzone, although it should be understood that the system could beconfigured to separately track the packages with respect to respectivedetection zones, particularly where the antennas are offset from eachother. Preferably, the antennas are disposed so that the detection zonesat least partially overlap. RF absorber pads are provided (e.g. as shownin FIG. 1). Interference antennas are not employed but could beprovided. Except as discussed herein, the construction and operation ofthe controller, engine(s) and tracking algorithm are the same asdiscussed with respect to FIGS. 1 and 2.

The tunnel's antennas are driven by a single RF engine 700 controlled byHSC 30. Engine 700 is comprised of the schematically illustratedcomponents of FIG. 39, including a microprocessor 100, transmitter 104,receiver 106, and FPGA 108. Transmitter 104 and receiver 106 connect toFPGA 108. Microprocessor 100 controls transmitter 104, receiver 106 andFPGA 108 and communicates with HSC 30 via connection line 103. Theengine includes four ports 800, 802, 804, and 806. Ports 800 and 802communicate with transmitter 104, while ports 804 and 806 communicatewith receiver 106. Each of transmitter 104 and receiver 106 include arespective switch (not shown) that switches transmitter 104 between port800 and port 802 and switches receiver 106 between port 804 and port806. HSC 30 initiates a simultaneous transmission by either of twoantenna pairs (76/56 or 46/66) by a command to microprocessor 100. Inresponse, microprocessor 100 sends a bit sequence and a switchinstruction to transmitter 104. The switch instruction causes thetransmitter to select either port 800 or port 802, depending upon theinstruction. As described in more detail below, whenever transmitter 104drives one of the two antenna pairs, the receiver should be set toreceive signals from that same antenna pair. That is, when the switch intransmitter 104 is set to port 800, the switch in receiver 106 should beset to port 804, and when the switch in transmitter 104 is set to port802, the switch in receiver 106 should be set to port 806. Accordingly,when microprocessor 100 sets the transmitter switch to either port 800or port 802, a signal from transmitter 104 (indicated at 808)automatically sets the switch in receiver 106 to the appropriate port.Alternatively, microprocessor 100 may send respective signals totransmitter 104 and receiver 106 to set the switches appropriately.

Assuming the transmitter and receiver are set to ports 800/804 or802/806, the transmitter transmits the signal at a specified frequencyand power level to the antenna pair via the specified antenna port. Oneor both of the antennas return detected signals from an RFID tag via theselected receiver port. Receiver 106 removes the carrier signal andsends the resulting information signal to FPGA 108. FPGA 108 extractsdigital data from the receiver's signal and outputs a resulting digitalsignal to microprocessor 100, which then transmits the digital signal toHSC 30. An RFID engine suitable for use in the presently disclosedembodiment is available from Symbol Technologies, Inc. of San Jose,Calif. (e.g. the Matrics AR400).

Transmitter 104 outputs through ports 800 and 802 to respective RFcirculators 808 and 810. The circulators are of an identicalconstruction. Accordingly, while the present discussion addressescirculator 808, it should be understood that circulator 810 has theidentical construction and function. Circulator 808 defines acounterclockwise signal circulation from the prospective shown in FIG.39. The transmitter signal provided to port 812 by transmitter port 800is output to a power splitter 814 by a port 816. The split signal frompower splitter 814 is output to respective bottom and side antennas 76and 56. The response signals returned by RFID tags and received by thesetwo antennas are combined at power splitter 814 and input to thecirculator at port 816. The circulator outputs the received signal fromport 818 to receiver 106 via receiver port 804. Transmitter 104 drivestop antenna 46 and side antenna 66, and receiver 106 receives signalsfrom these antennas, in the same manner through circulator 810 and powersplitter 820.

It is apparent that the system illustrated in FIG. 39 simultaneouslydrives, and simultaneously reads, multiple antennas, in this instancealternating pairs of antennas 76/56 and 46/66. Thus, for example, eachof top antenna 46 and right-side antenna 66 is activated to transmitquery signals and receive tag responses at the same time as the other.The antennas in the presently described embodiments are directional, andit is preferred that the centerlines of the gain pattern main lobes ofsimultaneously activated antennas are not coincident or parallel witheach other. Because the top antenna faces downward and the right-sideantenna faces horizontally, the centerlines of the two radiationpatterns are at least at a 90 degree angle with respect to each other.The radiation patterns are further offset from each other, however,because each of the two antennas is disposed at a 45 degree angle withrespect to the belt (and the antennas may be disposed so that theradiation centerlines do not cross). Thus, there is less likelihood thatthe radiation pattern from one of the two antennas will cause adetrimental level of noise in the other antenna. While there can be anincreased likelihood of insufficient antenna isolation if the two sideantennas were simultaneously activated, or if the top and bottomantennas were simultaneously activated, this risk is reduced by theangled orientation of the antennas with respect to the belt and mayotherwise be influenced by particular system conditions, such as powerlevel. In general, the antennas are disposed so that the gain pattern inone simultaneously activated antenna does not create sufficient noise inany other simultaneously activated antenna to prevent the antennas fromreceiving responses from the RFID tags at a rate that is acceptable forthe particular purposes of the system. Accordingly, while the presentembodiment simultaneously drives two antennas at a time, it should beunderstood that more than two antennas could be operated simultaneouslyand that combinations of simultaneously driven antennas other than theparticular example described herein may be employed.

To the extent the signals transmitted by antennas 76 and 56 have thesame power and are in phase with each other where they intersect, theradiation patterns tend to add to each other. Conversely, to the extentthey have the same power and are out of phase, the radiation patternstend to subtract from each other where they intersect. The extent towhich this occurs within the system's detection zone depends upon theantenna configuration, the disposition of the antennas, and the lengthsof the cables between the power splitter and the antennas. In general,there are points within the detection zone at which the signals from thesimultaneously-activated antennas entirely or to some extent cancel, andthere are points in the detection zone at which the signals add. Becausethe RFID tags move through the detection zone, they can be expected topass through addition zones as well as any cancellation zones that maybe present, with the result that the RFID tags may receive and respondto signals from antennas 76 and 56 despite the existence of cancellationareas within the detection zone.

Conversely, when an RFID tag responds to a query signal, the responsesignal typically reaches antennas 76 and 56 at slightly different times.This can cause some degree of interference when the signals from theantennas are combined at power splitter 814, but the degree ofinterference is generally insufficient to prevent reader 106 fromdetecting and reading the response signal.

In the arrangement illustrated in FIGS. 38 and 39, simultaneousactivation of two antennas at each antenna activation allows controller30 to execute, in effect, a two-antenna sequence rather than afour-antenna sequence. Thus, over a given period of time, each of thefour antennas is activated twice as often as in the embodiment describedabove with respect to FIGS. 1 and 2 (assuming that all four antennas aredriven in the embodiment discussed with respect to FIGS. 1 and 2),thereby increasing the probability that an RFID tag will be read as itpasses through the detection zone. One preferred sequence requiresactivation of each antenna pair to query for one tag class and then theother. Antenna sequence thread 204 instructs antenna engine 700 toselect transmitter and antenna ports 800 and 804, power antennas 76 and56 on, set their power level and frequencies to transmit to and receivesignals from class 0 RFID tags, transmit an RF query signal, and readany corresponding returned signals. Antenna sequence thread 204 theninstructs antenna engine 700 to change the power level and frequency ofantennas 76 and 56 to query and receive responses from class 1 tags.Antenna sequence thread 204 then instructs engine 700 to switch totransmitter and receiver ports 802 and 806 and attempt to read class 0and class 1 tags through antennas 46 and 66 in the same manner. Thelength of time each antenna pair is activated may be selected as part ofthe system's configuration. In one embodiment, the default activationtime is 13 milliseconds (applicable to each of the three tag protocolsdescribed herein, i.e. class 0, class 1 and Gen2). Read thread 708receives the responses received from the RFID tags and passes theinformation to tracking thread 200. The operation of the read andtracking threads, and the operation of the tracking algorithm ingeneral, is otherwise the same as in the embodiment described above withrespect to FIGS. 1 and 2 and is, therefore, not described furtherherein.

A scout reader (See FIG. 1) may again be provided to make upstreamqueries for tag type, and the scout thread may provide sufficientinstruction to sequence thread 204 so that the sequence threadcorrespondingly weights the queries to one tag type or another,depending on its prevalence. Because the top antenna 46 is always drivenin this example, second photodetector 40 may be omitted.

While one or more preferred embodiments of the invention have beendescribed, it should be understood that any and all equivalentrealizations of the present invention are included within the scope andspirit thereof. The embodiments depicted are presented by way of exampleonly and are not intended as limitations upon the present invention.Thus, it should be understood by those of ordinary skill in this artthat the present invention is not limited to these embodiments sincemodifications can be made. Therefore, it is contemplated that any andall such embodiments are included in the present invention.

1. A conveyor system for processing items on which radio frequencyidentification tags are disposed, said system comprising: a frame; aconveyor that is disposed movably on the frame and that conveys itemsthrough a path of travel, each item having at least one respective radiofrequency identification tag disposed thereon; an antenna disposed onthe frame with respect to the path of travel so that the antennaradiates radio frequency signals into a first area through which theitems pass, wherein the antenna comprises a substrate and a plurality ofpatch elements having respective generally planar surfaces and that aredisposed on the substrate in respective positions that are sequentialwith respect to a direction transverse to the path of travel, whereinthe generally planar surfaces of the patch elements are generallycoplanar, wherein the antenna includes a feed network that appliesrespective signals to each said patch element that drive electriccurrent at the patch elements to radiate the radio frequency signals,wherein the antenna is disposed with respect to the conveyor and thepath of travel so that respective said radio frequency identificationtags on said items are detected by a near field component of the radiofrequency signal radiated by one of the patch elements withoutinterference from the other patch elements, and wherein the respectivesignals applied by the feed network to at least two said patch elementsdefine a predetermined phase shift with respect to each other that isfixed so that respective electric current patterns on the at least twopatch elements are out of phase with each other; and a radio frequencytransmitter that drives the antenna to emit the radio frequency signalsinto the first area.
 2. The system as in claim 1, wherein the respectivesignals applied by the feed network to each pair of adjacent said patchelements define the predetermined phase shift with respect to eachother.
 3. The system as in claim 2, wherein the predetermined phaseshift between each pair of said adjacent patch elements is the same. 4.The system as in claim 3, wherein the predetermined phase shift betweeneach pair of said adjacent patch elements is approximately 90 degrees.5. The system as in claim 1, wherein the feed network applies therespective signals from a common source, and wherein the predeterminedphase shift is defined by lengths of traces in the feed network betweenthe common source and respective said patch elements.
 6. A conveyorsystem for processing items on which radio frequency identification tagsare disposed, said system comprising: a frame; a generally planar beltthat is disposed movably on the frame and that conveys items through apath of travel, each item having at least one respective radio frequencyidentification tag disposed thereon; an antenna disposed on the framebeneath the belt and comprising a substrate and a plurality of patchelements having respective generally planar surfaces and that aredisposed on the substrate in respective positions that are sequentialwith respect to a direction transverse to the path of travel, theantenna being oriented with respect to the belt so that the antennaradiates radio frequency signals into a first area that is proximate thebelt and through which the items pass, wherein the generally planarsurfaces of the patch elements are generally coplanar, wherein theantenna includes a feed network that applies respective signals to eachsaid patch element that drive electric current at the patch elements toradiate the radio frequency signals, wherein the antenna is disposedwith respect to the belt and the path of travel so that respective saidradio frequency identification tags on said items are detected by a nearfield component of the radio frequency signal radiated by one of thepatch elements without interference from the other patch elements, andwherein the respective signals applied by the feed network to at leasttwo said patch elements define a predetermined phase shift with respectto each other that is fixed so that respective electric current patternson the at least two patch elements are out of phase with each other; anda radio frequency transmitter that drives the antenna to emit the radiofrequency signals into the first area.
 7. The system as in claim 6,wherein the respective signals applied by the feed network to each pairof adjacent said patch elements define the predetermined phase shiftwith respect to each other.
 8. The system as in claim 7, wherein thepredetermined phase shift between each pair of said adjacent patchelements is the same.
 9. The system as in claim 8, wherein thepredetermined phase shift between each pair of said adjacent patchelements is approximately 90 degrees.
 10. The system as in claim 6,wherein the feed network applies the respective signals from a commonsource, and wherein the predetermined phase shift is defined by lengthsof traces in the feed network between the common source and respectivesaid patch elements.
 11. A conveyor system for processing items on whichradio frequency identification tags are disposed, said systemcomprising: a frame; a generally planar belt that is disposed movably onthe frame and that conveys items through a path of travel, each itemhaving at least one respective radio frequency identification tagdisposed thereon; an antenna disposed on the frame beneath the belt andcomprising a substrate and a plurality of patch elements havinggenerally planar surfaces and that are disposed on the substrate intandem with respect to a direction transverse to the path of travel, theantenna being oriented with respect to the belt so that the antennaradiates radio frequency signals into a first area that is proximate thebelt and through which the items pass and receives responses to theradio frequency signals from the respective radio frequencyidentification tags on the items on the belt, wherein the generallyplanar surfaces of the patch elements are generally coplanar, whereinthe antenna includes a feed network that applies respective signals toeach said patch element that drive electric current at the patchelements to radiate the radio frequency signals, wherein the antenna isdisposed with respect to the belt and the path of travel so thatrespective said radio frequency identification tags on said items aredetected by a near field component of the radio frequency signalradiated by one of the patch elements without interference from theother patch elements, and wherein the respective signals applied by thefeed network to each pair of adjacent said patch elements define apredetermined phase shift with respect to each other that is fixed sothat respective electric current patterns on the pair of adjacent patchelements are out of phase with each other; a radio frequency transmitterthat drives the antenna to emit the radio frequency signals into thefirst area; and a radio frequency receiver that receives signals fromthe antenna corresponding to the responses received by the antenna fromthe respective radio frequency identification tags and outputs firstsignals corresponding to the responses.
 12. The system as in claim 11,wherein the predetermined phase shift between each said pair of adjacentpatch elements is the same.
 13. The system as in claim 12, wherein thepredetermined phase shift is approximately 90 degrees.
 14. The system asin claim 11, wherein the feed network applies the respective signalsfrom a common source, and wherein the predetermined phase shift isdefined by lengths of traces in the feed network between the commonsource and respective said patch elements.
 15. A method of processingitems on which radio frequency identification tags are disposed, saidmethod comprising the steps of: providing a frame; providing a conveyordisposed movably on the frame that conveys items through a path oftravel, each item having at least one respective radio frequencyidentification tag disposed thereon; providing an antenna on the framewith respect to the path of travel so that the antenna radiates radiofrequency signals into a first area through which the items pass,wherein the antenna comprises a substrate and a plurality of patchelements having respective generally planar surfaces and that aredisposed on the substrate in respective positions that are sequentialwith respect to a direction transverse to the path of travel, whereinthe generally planar surfaces of the patch elements are generallycoplanar, and wherein the antenna includes a feed network that appliesrespective signals to each said patch element that drive electriccurrent at the patch elements to radiate the radio frequency signals;applying the respective signals by the feed network to at least two saidpatch elements, wherein the respective signals define a predeterminedphase shift with respect to each other that is fixed; providing a radiofrequency transmitter that drives the antenna to emit the radiofrequency signals into the first area so that respective electriccurrent patterns on the at least two patch elements are out of phasewith each other; and conveying items on the conveyor through the path oftravel so that respective said radio frequency identification tags onsaid items are detected by a near field component of the radio frequencysignals radiated by one of the patch elements without interference fromthe other patch elements.
 16. The method as in claim 15, wherein theapplying step comprises applying the respective signals by the feednetwork to each pair of adjacent said patch elements defining thepredetermined phase shift with respect to each other.
 17. The method asin claim 16, wherein the predetermined phase shift between each pair ofsaid adjacent patch elements is the same.
 18. The method as in claim 15,wherein the feed network applies the respective signals from a commonsource, and wherein the predetermined phase shift is defined by lengthsof traces in the feed network between the common source and respectivesaid patch elements.